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Sudhir K. Pandey

Publications and source records attributed to Sudhir K. Pandey.

At least 73 records · Page 4Linked to original sources
arXiv2018

Automated Instrumentation for the Determination of the High-Temperature Thermoelectric Figure-of-Merit

In this work, we report the fabrication of a high temperature measurement setup to measure Figure of merit (ZT). This setup facilitates the simultaneous measurement of Seebeck coefficient (α), thermal conductivity (κ), and electrical resistivity (ρ) required to calculate ZT. Measurement of temperature, as well as voltages using same thermocouples, simplified the design of the setup by minimizing sensors and wires. Limited components used in the sample holder further simplify the design and make it small in size and lightweight. The dedicated thin heater is made, which minimizes the heat loss. Further, low heat loss is achieved by optimizing the insulator dimension. To measure power delivered to the heater, 4-wire technique is used. Low cost and commonly available materials used in the fabrication of various components make it more accessible to the user as any parts can be easily replaced in case of any damage occurs. A dedicated program is built in the Python programming language to automate the whole measurement process. p-type Bi0.36Sb1.45Te3 sample is used to calibrate this measurement setup. The data collected are found to be in good agreement with the reported data.

physics.ins-det↗
arXiv2017

Studying the occupied and unoccupied electronic structure of LaCoO$_{3}$ by using DFT+embedded DMFT method with the calculated value of $\textit{U}$

In this work, we present a systematic study of the occupied and unoccupied electronic states of LaCoO$_{3}$ compound using DFT, DFT+$\textit{U}$ and DFT+embedded DMFT methods. The value of $\textit{U}$ used here is evaluated by using constrained DFT method and found to be $ \backsim $ 6.9 eV. It is found that DFT result has limitations with energy positions of PDOS peaks due to its inability of creating a hard gap although the DOS distribution appears to be fine with experimental attributes. The calculated value of $\textit{U}$ is not an appropriate value for carrying out DFT+$\textit{U}$ calculations as it has created an insulating gap of $ \backsim $ 1.8 eV with limitations in redistribution of DOS which is inconsistent with experimental spectral behaviour for the occupied states mainly. However, this value of $\textit{U}$ is found to be an appropriate one for DFT+embedded DMFT method which creates a gap of $\backsim $ 1.1 eV. The calculated PDOS of Co 3$\textit{d}$, La 5$\textit{d}$, La 4$\textit{f}$ and O 2$\textit{p}$ states are giving a remarkably good explanation for the occupied and unoccupied states of the experimental spectra in the energy range $\backsim $ -9.0 eV to $\backsim $ 12.0 eV.

cond-mat.str-el↗
arXiv2017

Theoretical Study of Thermopower Behavior of LaFeO$_{3}$ Compound in High Temperature Region

The electronic structure and thermopower ($α$) behavior of LaFeO$_{3}$ compound were investigated by combining the ab-initio electronic structures and Boltzmann transport calculations. LSDA plus Hubbard U (U = 5 eV) calculation on G-type anti-ferromagnetic (AFM) configuration gives an energy gap of $\sim$2 eV, which is very close to the experimentally reported energy gap. The calculated values of effective mass of holes (m$^{*}$$_{h}$) in valance band (VB) are found $\sim$4 times that of the effective mass of electrons (m$^{*}$$_{e}$) in conduction band (CB). The large effective masses of holes are responsible for the large and positive thermopower exhibited by this compound. The calculated values of $α$ using BoltzTraP code are found to be large and positive in the 300-1200 K temperature range, which is in agreement with the experimentally reported data.

cond-mat.str-el↗
arXiv2017

A comparative study of different exchange-correlation functionals in understanding structural, electronic and thermoelectric properties of Fe$_{2}$VAl and Fe$_{2}$TiSn compounds

Fe$_{2}$VAl and Fe$_{2}$TiSn are full Heusler compounds with non-magnetic ground state. The two compouds are good thermoelectric materials. PBE and LDA(PW92) are the two most commonly used density functionals to study the Heusler compounds. Along with these two well studied exchange-correlation functionals, recently developed PBEsol, mBJ and SCAN functionals are employed to study the two compounds. Using the five functionals equilibrium lattice parameter and bulk modulus are calculated. Obtained values are compared with experimental reports wherever available. Electronic structure properties are studied by calculating dispersion curves, total and partial density of states. For Fe$_{2}$VAl, band gap of 0.22 eV is obtained from the mBJ potential which is in reasonable agreement with experimental value while, for Fe$_{2}$TiSn band gap of 0.68 eV is obtained. Fe$_{2}$VAl is predicted to be semimetallic with different values of negative gaps from LDA,PBEsol,PBE and SCAN functionals. Whereas, Fe$_{2}$TiSn is found to be semimetallic(semiconducting) from LDA,PBEsol(PBE,SCAN) functionals employed calculations. From the dispersion curve effective mass values are also computed to see the contribution to the Seebeck coefficient. In Fe$_{2}$TiSn, a flat band is present along the $Γ$-X direction with calculated value of effective mass $\sim$36 more than the mass of electron. The improvements or inadequacies among the functionals in explaining the properties of full Heusler alloys for thermoelectric application are thus observed through this study.

cond-mat.str-el↗
arXiv2017

Studying the effect of different exchange correlation functionals on the structural and electronic properties of a half-Heusler NaAuS compound

Theoretically, NaAuS is predicted as topological insulator, while no detail electronic structure study has been done for this compound. Here, we report the structural and electronic properties of NaAuS by using LDA, PBEsol, PBE and revPBE exchange correlation functionals. The calculated values of equilibrium lattice constant for LDA, PBEsol, PBE and revPBE exchange correlation functionals are found to be $\sim$6.128 Å, $\sim$6.219 Å, $\sim$6.353 Å and $\sim$6.442 Å, respectively. The bulk modulus predicted by LDA, PBEsol, PBE and revPBE exchange correlation functionals is $\sim$66.6, $\sim$56.4, $\sim$46.5 and $\sim$39.3 GPa, respectively. Hence, the order of calculated values of bulk modulus is consistent with the order of calculated values of equilibrium lattice parameters for these exchange correlation functionals. The spread of total density of states below the Fermi level decreases as the exchange correlation functional changes from LDA to PBEsol to PBE to revPBE, which is also found to be consistent with the order of bulk modulus for these exchange correlation functionals. In presence of spin-orbit coupling, a direct band gap is observed in NaAuS compound, which is found to be $\sim$0.26, $\sim$0.25, $\sim$0.24 and $\sim$0.23 eV for LDA, PBEsol, PBE and revPBE exchange correlation functionals, respectively. Here, NaAuS is found to be topological insulator as it shows band inversion at $Γ$ point. The calculated values of band inversion strength for LDA (PBEsol) and PBE (revPBE) exchange correlation functionals are $\sim$1.58 eV ($\sim$1.57 eV) and $\sim$1.50 eV ($\sim$1.47 eV), respectively.

cond-mat.str-el↗
arXiv2017

Fabrication of a simple apparatus for the Seebeck coefficient measurement in high temperature region

A simple apparatus for the measurement of Seebeck coefficient ($α$) in the temperature range 300-620 K has been fabricated. Our design is appropriate for the characterization of samples with different geometries like disk and rod shaped. The sample holder assembly of the apparatus has been designed in such a way that, single heater used for sample heating purpose is enough to provide a self maintain temperature gradient (1-10 K) across the sample. The value of $α$ is obtained without explicit measurement of temperature gradient. The whole apparatus is fabricated from the materials, which are commonly available, so that any part can be replaced in case of any damage. Commercially available standard Nickel (Ni) metal sample has been used as a reference material for calibration of the instrument. The experimentally observed value of $α$ by our apparatus gives the similar temperature dependent behavior as reported in the literature. In order to study the thermoelectric behavior of oxide materials, we have synthesized polycrystalline LaCoO$_{3}$ powder samples by using solution combustion method and investigated the thermoelectric properties. Temperature dependent thermoelectric behavior of these samples were characterized in the temperature range 300-600 K.

physics.ins-det↗
arXiv2017

Understanding the Thermoelectric Properties of LaCoO$_{3}$ Compound

We present the thermoelectric (TE) properties of LaCoO$_{3}$ compound in the temperature range 300-600 K. The experimental value of Seebeck coefficient ($α$) at 300 K is found to be $\sim$635 $μ$V/K. The value of $α$ decreases continuously with increase in temperature and reaches to $\sim$46 $μ$V/K at $\sim$600 K. The electronic and TE properties of the compound have also been investigated by combining the \textit{ab-initio} electronic structures and Boltzmann transport calculations. LSDA plus Hubbard U (U= 2.75 eV) calculation on low spin configuration of the compound gives an energy gap of $\sim$0.5 eV, which is close to the experimentally reported energy gap. The effective mass of holes (\textit{m$^{*}_h$}) at $Γ$ point is nearly two times larger than the value of effective mass of electrons (\textit{m$^{*}_e$}) at FB point along the L and T directions, whereas the \textit{m$^{*}_e$} at FB point along the $Γ$ direction is nearly eight times larger than the value of \textit{m$^{*}_h$} at $Γ$ point along the FB direction. The large effective mass at FB point along the $Γ$ direction suggests that the TE property of this compound is mainly decided by the effective mass of the charge carriers in this direction. The calculated temperature dependent values of $α$ are in fairly good agreement with experimental data in the temperature range 300-360 K, and above this temperature slight deviation is observed. The value of power factor (PF) for \textit{n}-type is $\sim$1.3 times larger the value of \textit{p}-type doped compound at 1100 K. The value of \textit{figure-of-merit} (\textit{ZT}) for \textit{n}-type doped compound is obtained $\sim$0.35 in the temperature range 600-1100 K, which suggests that with appropriate \textit{n}-type doping this compound can be used as a good TE material in the high temperature region.

cond-mat.str-el↗
arXiv2017

An important role of temperature dependent scattering time in understanding the high temperature thermoelectric behavior of strongly correlated system: La$_{0.75}$Ba$_{0.25}$CoO$_{3}$

In the present work, we have reported the temperature dependent thermopower ($α$) behavior of La$_{0.75}$Ba$_{0.25}$CoO$_{3}$ compound in the temperature range 300-600 K. Using the Heikes formula, the estimated value of $α$ corresponding to high-spin configuration of Co$^{3+}$ and Co$^{4+}$ ions is found to be $\sim$$16$ $μ$V/K, which is close to the experimental value, $\sim$13 $μ$V/K, observed at $\sim$600 K. The temperature dependent TE behavior of the compound is studied by combining the WIEN2K and BoltzTrap code. The self consistency field calculations show that the compound have ferromagnetic ground state structure. The electronic structure calculations give half metallic characteristic with a small gap of $\sim$50 meV for down spin channel. The large and positive value for down spin channel is obtained due to the unique band structure shown by this spin channel. The temperature dependent relaxation time for both the spin-channel charge carriers is considered to study the thermopower data in temperature range 300-600 K. An almost linear values of $τ_{up}$ and a non-linear values of $τ_{dn}$ are taken into account for evaluation of $α$. By taking the temperature dependent values of relaxation time for both the spin channels, the calculated values of $α$ using two current model are found to be in good agreement with experimental values in the temperature range 300-600 K. At 300 K, the calculated value of electrical conductivity by using the same value of relaxation time, i.e. 0.1 $\times $10$^{-14}$ seconds for spin-up and 0.66 $\times $10$^{-14}$ seconds for spin-dn channel, is found to be equal to the experimentally reported value.

cond-mat.str-el↗
arXiv2017

Fabrication of fully automated setup for high temperature thermal conductivity measurement

In this work, we report the fabrication of experimental setup for high temperature thermal conductivity ($κ$) measurement. It can characterize samples with various dimensions and shapes. Steady state based axial heat flow technique is used for $κ$ measurement. Heat loss is measured using parallel thermal conductance technique. Simple design, lightweight and small size sample holder is developed by using a thin heater and limited components. Low heat loss value is achieved by using very low thermal conductive insulator block with small cross sectional area. Power delivered to the heater is measured accurately by using 4-wire technique and for this, heater is developed with 4-wire. This setup is validated by using $Bi_{0.36}Sb_{1.45}Te_3$, polycrystalline bismuth, gadolinium, and alumina samples. The data obtained for these samples are found to be in good agreement with the reported data. The maximum deviation of 6 \% in the value $κ$ are observed. This maximum deviation is observed with the gadolinium sample. We also report the thermal conductivity of polycrystalline tellurium from 320 K to 550 K and the non monotonous behavior of $κ$ with temperature is observed.

physics.ins-det↗
arXiv2017

Fabrication of Simple Apparatus for Resistivity Measurement in High Temperature Range 300-620 K

A simple and low cost apparatus has been designed and built to measure the electrical resistivity, ($ρ$), of metal and semiconductors in 300-620 K temperature range. The present design is suitable to do measurement on rectangular bar sample by using conventional four-probe dc method. A small heater is made on the sample mounting copper block to achieve the desired temperature. Heat loss from sample holder is minimize by using very low thermal conductive insulator block. This unique design of heater and minimized heat loss from sample platform provide uniform sample temperature and also have very good thermal stability during the measurement. The electrical contacts of current leads and potential probes on the sample are done by using very thin (42 SWG) copper wires and high temperature silver paste. The use of limited components and small heater design make present instrument very simple, light weight, easy to sample mount, small in size, and low cost. To calibrate the instrument pure nickel sample was used, and two other materials La$_{0.7}$Sr$_{0.3}$MnO$_{3}$ (LSMO) and LaCoO$_{3}$ (LCO) were also characterized to demonstrate the accuracy of this set-up. $ρ$(T) behavior on these samples were found to be in good agreement with the reported data. The metal-insulator transition for LSMO (T$_{MI}$ = $\sim$358 K) and the insulator-metal transition for LCO (T$_{IM}$ = $\sim$540 K) were clearly observed and these transitions temperature were also consistent with those reported in literature.

physics.gen-ph↗
arXiv2017

Understanding the High Temperature Thermoelectric Properties of La$_{0.82}$Ba$_{0.18}$CoO$_{3}$ Compound using DFT+U Method

Normally, understanding the temperature dependent transport properties of strongly correlated electron systems remains challenging task due to complex electronic structure and its variations (around E$_{F}$) with temperature. Here, we report the applicability of DFT+U in explaining thermopower ($α$) and electrical conductivity ($σ$) in high temperature region. We have measured temperature dependent $α$ and $σ$ in the 300-600 K range. The non-monotonic temperature dependent behavior of $α$ and metallic behavior of $σ$ were observed. The value of $α$ at 300 K was $\sim$15.80 $μ$V/K and it decreases upto $\sim$477 K ($\sim$11.6 $μ$V/K) and it further increases with temperature to the $\sim$14.8 $μ$V/K at 600 K, whereas the values of $σ$ were found to be $\sim$1.42 $\times$10$^{5}$ $Ω$$^{-1}$ m$^{-1}$ and $\sim$0.20 $\times$10$^{5}$ $Ω$$^{-1}$ m$^{-1}$ at 300 and 600 K, respectively. Combining the WIEN2k and BoltzTraP code, the electronic structure and temperature dependent transport coefficients were calculated. The ferromagnetic ground state electronic structure with half-metallic character obtained from the DFT+U calculations, U = 3.1 eV, provides better explanation of high-temperature transport behavior. Two current model was used for calculation of $α$ and $σ$ where the temperature dependent values of relaxation time ($τ$), almost linear for up-spin, $τ$$_{up}$, and non-linear for dn-spin, $τ$$_{dn}$, were used and estimated values were found to be in good agreement with experimentally reported values.

cond-mat.str-el↗
arXiv2017

Electronic structure study of vanadium spinels by using density functional theory and dynamical mean field theory

Theoretically, various physical properties of AV$_{2}$O$_{4}$ (A=Zn, Cd and Mg) spinels have been extensively studied for last 15 years. Besides of this, no systematic comparative study has been done for these compounds, where the material specific parameters are used. Here, we report the comparative electronic behaviour of these spinels by using a combination of density functional theory and dynamical mean-field theory, where the self-consistent calculated Coulomb interaction $U$ and Hund's coupling $J$ (determined by Yukawa screening $λ$) are used. The main features, such as insulating band gaps ($E_{g}$), degree of itinerancy of V 3$d$ electrons and position of lower Hubbard band are observed for these parameters in these spinels. The calculated values of $E_{g}$ for ZnV$_{2}$O$_{4}$, CdV$_{2}$O$_{4}$ and MgV$_{2}$O$_{4}$ are found to be $\sim$0.9 eV, $\sim$0.95 eV and $\sim$1.15 eV, respectively, where the values of $E_{g}$ are close to experiment for ZnV$_{2}$O$_{4}$ and MgV$_{2}$O$_{4}$. The position of lower Hubbard band are observed around $\sim$-1.05 eV, $\sim$-1.25 eV and $\sim$-1.15 eV for ZnV$_{2}$O$_{4}$, CdV$_{2}$O$_{4}$ and MgV$_{2}$O$_{4}$, respectively, which are also in good agreement with the experimental data for ZnV$_{2}$O$_{4}$. The order of average impurity hybridization function of V site are found to be ZnV$_{2}$O$_{4}$$>$MgV$_{2}$O$_{4}$$>$CdV$_{2}$O$_{4}$. Hence, the degree of localization of V 3$d$ electrons is largest for CdV$_{2}$O$_{4}$ and smallest for ZnV$_{2}$O$_{4}$, which is in accordance with our earlier results. Hence, present work shows the importance of material specific parameters to understand the comparative electronic behaviour of these compounds.

cond-mat.str-el↗
arXiv2017

Interfacing of High Temperature Z-meter Setup Using Python

In this work, we interface high temperature Z-meter setup to automize the whole measurement process. A program is built on open source programming language Python which convert the manual measurement process into fully automated process without any cost addition. Using this program, simultaneous measurement of Seebeck coefficient, thermal conductivity and electrical resistivity are performed and using all three, figure-of-merit (ZT) is calculated. Developed program is verified by performing measurement over p-type Bi0.36Sb1.45Te3 sample and the data obtained are found to be in good agreement with the reported data.

physics.ins-det↗
arXiv2016

Automated Instrumentation for High-Temperature Seebeck Coefficient Measurements

In this work, we report the fabrication of fully automated experimental setup for high temperature Seebeck coefficient (α) measurement. The K-type thermocouples are used to measure the average temperature of the sample and Seebeck voltage across it. The temperature dependence of the Seebeck coefficients of the thermocouple and its negative leg is taken care by using the integration method. Steady state based differential technique is used for α measurement. Use of limited component and thin heater simplify the sample holder design and minimize the heat loss. The power supplied to the heater decides the temperature difference across the sample and the measurement is carried out by achieving the steady state. The LabVIEW based program is built to automize the whole measurement process. The complete setup is fabricated by using commonly available materials in the market. This instrument is standardized for materials with a wide range of α and for the wide range of ΔT across the specimen. For this purpose, high temperature measurement of α of iron, constantan, bismuth, and Bi_0.36 Sb_1.45 Te_3 samples are carried out and data obtained from these samples are found to be in good agreement with the reported data.

physics.ins-det↗
arXiv2016

Role of orbital degrees of freedom in investigating the magnetic properties of geometrically frustrated vanadium spinels

The inconsistency about the degree of geometrical frustration has been a long issue in AV$_{2}$O$_{4}$ (A $\equiv$ Zn, Cd and Mg) compounds, which arises from the two experimental results: (i) frustration indices and (ii) magnetic moments. In the present study, we try to understand such inconsistency by using {\it ab initio} electronic structure calculations. The orbital degrees of freedom are found to play an important role in understanding the geometrically frustrated magnetic behaviour of these compounds. The inclusion of the orbital and spin angular momenta for calculating the frustration indices improves the understanding about the degree of geometrical frustration in these compounds. The calculated values of the frustration indices ($f$$_{\it J}$) are largest for MgV$_{2}$O$_{4}$ and smallest for CdV$_{2}$O$_{4}$ for 3.3$\leq$ $U \leq$5.3 eV. In this range of $U$, the calculated values of $Δ$M$_{2}$=M$_{\rm total}$-M$_{\rm exp}$ are largest for MgV$_{2}$O$_{4}$ and smallest for CdV$_{2}$O$_{4}$. Hence, the consistency about the degree of geometrical frustration is achieved. The absolute values of the nearest neighbour exchange coupling constant ({\it J$_{nn}$}) between V spins are found to be largest for MgV$_{2}$O$_{4}$ and smallest for CdV$_{2}$O$_{4}$, which indicate that the calculated absolute values of the Curie-Weiss temperature ($\varTheta$$_{CW}$)$_{\it J}$ are highest for MgV$_{2}$O$_{4}$ and smallest for CdV$_{2}$O$_{4}$ for 3.3$\leq$ $U \leq$5.3 eV. In this range of $U$, the magnetic transition temperature ($T$$_{N}$)$_{\it J}$ is found to be $\sim$150 K, $\sim$60 K and $\sim$22 K for MgV$_{2}$O$_{4}$, ZnV$_{2}$O$_{4}$ and CdV$_{2}$O$_{4}$, respectively, which shows that the order of ($T$$_{N}$)$_{\it J}$ is similar to that of ($T$$_{N}$)$_{\rm exp}$ for these compounds.

cond-mat.str-el↗
arXiv2016

Self-consistent evaluation of effective Coulomb interaction of V atom and its importance to understand the comparative electronic behaviour of vanadium spinels

In present work, we try to understand the importance of effective Coulomb interaction ($U_{ef}$) between localized electrons of V atom to understand the comparative electronic behaviour of AV$_{2}$O$_{4}$ (A=Zn, Cd and Mg) compounds. The suitable values of $d$-linearization energy ($E_{d}$) of impurity V atom for calculating the $U_{ef}$ for these compounds are found to be $\geq$44.89 eV above the Fermi level. Corresponding to these values of $E_{d}$, the self-consistently calculated values of effective $U_{LSDA}$ ($U_{PBEsol}$) for ZnV$_{2}$O$_{4}$, MgV$_{2}$O$_{4}$ and CdV$_{2}$O$_{4}$ are $\sim$5.73 ($\sim$5.92), $\sim$6.06 ($\sim$6.22) and $\sim$5.59 ($\sim$5.71) eV, respectively. The calculated values of $\frac{t}{U_{ef}}$ ($t$ is the transfer integral between neighbouring sites) increases with decreasing V-V distance from CdV$_{2}$O$_{4}$ to MgV$_{2}$O$_{4}$ to ZnV$_{2}$O$_{4}$ and are found to be consistent with experimentally reported band gap. The values of $\frac{t}{U_{ef}}$ for ZnV$_{2}$O$_{4}$, MgV$_{2}$O$_{4}$ and CdV$_{2}$O$_{4}$ are found to be $\sim$0.023, $\sim$0.020 and $\sim$0.018, respectively. Hence, CdV$_{2}$O$_{4}$ with small (large) $\frac{t}{U_{ef}}$ (experimental band gap) as compared to ZnV$_{2}$O$_{4}$ and MgV$_{2}$O$_{4}$ is found to be in localized-electron regime, while ZnV$_{2}$O$_{4}$ and MgV$_{2}$O$_{4}$ are intermediate between localized and an itinerant-electron regime. The calculated values of lattice parameters $a_{LSDS}$ ($a_{PBEsol}$) are found to be $\sim$1.7\%, $\sim$2.0\% and $\sim$2.4\% ($\sim$0.6\%, $\sim$0.7\% and $\sim$0.7\%) smaller than $a_{exp}$ for CdV$_{2}$O$_{4}$, MgV$_{2}$O$_{4}$ and ZnV$_{2}$O$_{4}$, respectively, which indicates that the PBEsol functional predicts the lattice parameters in good agreement with the experimental data.

cond-mat.str-el↗
arXiv2016

The role of ionic sizes in inducing the cubic to tetragonal distortion in AV$_{2}$O$_{4}$ and ACr$_{2}$O$_{4}$ (A=Zn, Mg and Cd) compounds

Cubic to tetragonal distortion in spinel compounds have been a contentious issue for last two decades. Different groups have proposed different mechanisms to understand such a distortion in these spinels, which are: (i) spin lattice coupling mechanism known as the spin driven Jahn-Teller (JT) effect, (ii) the strong relativistic spin-orbit coupling, a moderate JT distortion and weak V-V interactions and (iii) the JT effect. Now, in order to know the possible cause for such a distortion, we have avoided these complexities (various interactions among spin, electronic, orbital and lattice degrees of freedom) by carrying out spin unpolarized calculations. The calculated values of bulk moduli for ZnV$_{2}$O$_{4}$ (ZnCr$_{2}$O$_{4}$), MgV$_{2}$O$_{4}$ (MgCr$_{2}$O$_{4}$) and CdV$_{2}$O$_{4}$ (CdCr$_{2}$O$_{4}$) are found to be $\sim$289 ($\sim$254), $\sim$244 ($\sim$243) and $\sim$230 ($\sim$233) GPa, respectively. For vanadates and chromates, the order of calculated values of lattice parameter $a$ are found to CdV$_{2}$O$_{4}$$>$MgV$_{2}$O$_{4}$$>$ZnV$_{2}$O$_{4}$ and CdCr$_{2}$O$_{4}$$>$MgCr$_{2}$O$_{4}$$>$ZnCr$_{2}$O$_{4}$, respectively and are consistent with the experimental results. The calculated values of cubic to tetragonal distortion (c/a), with c/a$<$1 for ZnV$_{2}$O$_{4}$ (ZnCr$_{2}$O$_{4}$), MgV$_{2}$O$_{4}$ (MgCr$_{2}$O$_{4}$) and CdV$_{2}$O$_{4}$ (CdCr$_{2}$O$_{4}$) are $\sim$0.996 ($\sim$0.997), $\sim$0.995 ($\sim$0.994) and $\sim$0.997 ($\sim$0.998), respectively. These values are in good agreement with the experimental data for ZnV$_{2}$O$_{4}$, MgV$_{2}$O$_{4}$, ZnCr$_{2}$O$_{4}$ and MgCr$_{2}$O$_{4}$ compounds. The present study clearly shows the role of ionic sizes in inducing the cubic to tetragonal distortion in these spinels. These mechanisms also appear to be responsible for deciding the other physical properties of these compounds.

cond-mat.str-el↗
arXiv2016

The importance of temperature dependent energy gap in the understanding of high temperature thermoelectric properties

In the present work, we show the importance of temperature dependent energy band gap, E$_{g}$(T), in understanding the high temperature thermoelectric (TE) properties of material by considering LaCoO$_{3}$ (LCO) and ZnV$_{2}$O$_{4}$ (ZVO) compounds as a case study. For the fix value of band gap, E$_{g}$, deviation in the values of $α$ has been observed above 360 K and 400 K for LCO and ZVO compounds, respectively. These deviation can be overcomed by consideration of temperature dependent band gap. The change in used value of E$_{g}$ with respect to temperature is $\sim$4 times larger than that of InAs. This large temperature dependence variation in E$_{g}$ can be attributed to decrement in the effective on-site Coulomb interaction due to lattice expansion. At 600 K, the value of \textit{ZT} for \textit{n} and \textit{p-doped}, LCO is $\sim$0.35 which suggest that it can be a used as a potential material for TE device. This work clearly suggest that one should consider the temperature dependent band gap in predicting the high temperature TE properties of insulating materials.

cond-mat.str-el↗