arXiv subjects

Rajesh Jana

Publications and source records attributed to Rajesh Jana.

9 recordsLinked to original sources

arXiv2026

Pressure-driven vibrational and structural peculiarities in the honeycomb layered magnetoelectrics Mn4(B)2O9 (B= Nb, Ta)

The high-pressure behavior of two Mn-based honeycomb-structured magnetoelectric materials, Mn4Nb2O9 (MNO) and Mn4Ta2O9 (MTO), was investigated using Raman spectroscopy, synchrotron x-ray diffraction, and density functional theory (DFT) calculations. In MTO, the application of a small pressure of only 0.5 GPa induces an isostructural transition driven by local symmetry breaking. With further increase in pressure, three additional isostructural transitions are observed at about 3.2, 6, and 10 GPa, followed by the onset of a long-range structural transition near 14 GPa, where the ambient P-3c1 phase begins to transform into a P2/c phase. These two phases coexist up to 27 GPa. The Nb analogue, MNO, also exhibits similar isostructural transitions at about 2, 6.6, and 10 GPa. However, the onset of the mixed P2/c and P-3c1 phases occurs at a slightly lower pressure of 12.5 GPa, with phase coexistence extending up to 26.5 GPa. These long-range transitions are supported by pressure-dependent enthalpy changes obtained from DFT calculations. Rietveld refinement reveals pronounced anisotropic lattice compression, with a 42 to 49 percent difference between the c and a axes, leading to a notable reduction in the c/a ratio. This anisotropy may strengthen interlayer coupling and promote magnetic ordering under compression, consistent with the appearance of Raman modes similar to those reported at low temperatures, together with anomalous changes in Raman mode linewidth and intensity. The marked changes in Raman self-energy parameters, anomalies in the reduced pressure-Eulerian strain profile, and the onset of local symmetry breaking at much lower pressures in MTO than in MNO highlight the important role of differences in spin-orbit coupling strength and orbital hybridization associated with Nb5+ and Ta5+ cations.

cond-mat.mtrl-sci↗

arXiv2026

Pressure-induced lattice instabilities and phonon softening in the orthorhombically distorted ferrimagnet Ni4Nb2O9

The ambient- and high-pressure behavior of the ferrimagnet Ni4Nb2O9 (orthorhombically distorted honeycomb structure), is investigated using NMR, Raman spectroscopy, and synchrotron XRD. Ambient-pressure NMR measurements reveal, despite its orthorhombic symmetry, the local environment of Ni4Nb2O9 closely resembles that of its trigonal analogue Mn4Nb2O9. In contrast, substantially different paramagnetic shifts observed in the two compounds reflect their distinct average crystal symmetries, governing orbital overlap and magnetic exchange pathways. Under external pressure, Ni4Nb2O9 exhibits pronounced sensitivity to lattice distortions and phonon instabilities. Three isostructural transitions are identified near 2, 6, and 10 GPa, manifested by mode splitting, frequency shifts, line broadenings, intensity anomalies, and slope changes in the evolution of lattice parameters. At higher pressure, around 13 GPa, signatures of an incipient long-range structural transition from orthorhombic Pbcn to monoclinic P2/c symmetry emerge, signaling the onset of a symmetry-lowering transformation. The anomalous softening of the 192 cm^-1 Raman mode, accompanied by multiple linewidth and spectral-weight anomalies, serve as a key fingerprint of these structural instabilities, linking local symmetry breaking at low pressures to the long-range transition into the P2/c phase. Notably, pronounced linewidth anomalies, strongly anisotropic pressure coefficients, together with a marked enhancement of the intensity of the low-frequency branch over the 2-13 GPa range, point toward a pressure-induced regime influenced by coupled spin, orbital, and lattice degrees of freedom. The close correspondence of transition pressures in Ni4Nb2O9 and those reported for Mn4Nb2O9 highlights a common mechanism rooted in their similar local structural environments, as revealed by NMR.

cond-mat.mtrl-sci↗

arXiv2026

Observation of robust spin-phonon coupling and indication of hidden structural transition in the spin-driven ferroelectrics Mn4B_2O_9 (B= Nb, Ta)

We report detailed Raman spectroscopic and magnetic susceptibility studies on the spin-driven ferroelectric compounds Mn4Nb2O9 (MNO) and Mn4Ta2O9 (MTO). Both systems exhibit strong spin-phonon coupling below the short-range magnetic ordering temperature (T(sro)=223 K), followed by further renormalization of several Raman modes at the long-range magnetic ordering temperatures (TN = 120 K for MNO and 110 K for MTO). Pronounced anomalies in Raman mode frequencies and linewidths, along with the emergence of octahedral modes between Tsro and TN, indicate a possible low-symmetry structural transition, more evident in MNO and closely linked to magnetic ordering in MTO. Distinct low-temperature evolutions of Raman mode shift, linewidth, and integrated intensity in MNO and MTO highlight the role of the nonmagnetic B-site cation in tuning spin-lattice coupling, driven by differences in spin-orbit coupling and orbital hybridization between Nb5+ (4d) and Ta5+ (5d). By combining Raman spectroscopy with nuclear magnetic resonance, and diffuse reflectance spectroscopy, we further show that Mn-based systems possess a more distorted local structure than their Co analogues, while their electronic structures differ despite comparable band gaps. These results provide a comprehensive understanding of spin-lattice coupling in Mn- and Co-based A4B2O9 magnetoelectric systems.

cond-mat.mtrl-sci↗

arXiv2026

muT2-NMR: Micro-Scale Correlation Relaxometry for in-situ High-Pressure Nuclear Magnetic Resonance

Over the last decade, frequency-domain in-situ high-pressure nuclear magnetic resonance (NMR) spectroscopy in diamond anvil cells (DACs) has been employed as a structural and electronic probe of condensed matter systems at pressures well into the megabar range. However, extensive spin interactions and sample heterogeneities under pressure often lead to significant spectral overlap, inhibiting independent observation of chemically similar spin sub-species in the same sample. In this work, we introduce a time-domain relaxometry framework specifically suited for DAC experiments, named muT2-NMR. Experimental flexibility and operational robustness are benchmarked on three hydrogen-rich molecular solids at pressures up to 72 GPa. We demonstrate that muT2-NMR can resolve individual molecular subunits in relaxation space, paving the way for novel high-pressure, high-resolution NMR applications in molecular solids.

physics.ins-det↗

arXiv2024

Diffusion Driven Transient Hydrogenation in Metal Superhydrides at Extreme Conditions

In recent years, metal hydride research has become one of the driving forces of the high-pressure community, as it is believed to hold the key to superconductivity close to ambient temperature. While numerous novel metal hydride compounds have been reported and extensively investigated for their superconducting properties, little attention has been focused on the atomic and electronic states of hydrogen, the main ingredient in these novel compounds. Here, we present combined $^{1}H$- and $^{139}La$-NMR data on lanthanum superhydrides, $LaH_{x}$, ($x = 10.2 - 11.1$), synthesized after laser heating at pressures above 160 GPa. Strikingly, we found hydrogen to be in a highly diffusive state at room temperature, with diffusion coefficients in the order of $10^{-6}~cm^2s^{-1}$. We found that this diffusive state of hydrogen results in a dynamic de-hydrogenation of the sample over the course of several weeks, approaching a composition similar to its precursor materials. Quantitative measurements demonstrate that the synthesized superhydrides continuously decompose over time. Transport measurements underline this conclusion as superconducting critical temperatures were found to decrease significantly over time as well. This observation sheds new light on formerly unanswered questions on the long-term stability of metal superhydrides.

cond-mat.mtrl-sci↗

arXiv2024

Hexagonal to Monoclinic Phase Transition in Dense Hydrogen Phase III Detected by High-Pressure NMR

Conclusive crystal structure determination of the high pressure phases of hydrogen remains elusive due to lack of core electrons and vanishing wave vectors, rendering standard high-pressure experimental methods moot. Ab-initio DFT calculations have shown that structural polymorphism might be solely resolvable using high-resolution nuclear magnetic resonance (NMR) spectroscopy at mega-bar pressures, however technical challenges have precluded such experiments thus far. Here, we present in-situ high-pressure high-resolution NMR experiments in hydrogen phase III between 181 GPa and 208 GPa at room temperature. Our spectra suggest that at lower pressures phase III adopts a hexagonal P6122 crystal structure, transitioning into a monoclinic C2/c phase at about 197 GPa. The high resolution spectra are in excellent agreement with earlier structural and spectral predictions and underline the possibility of a subtle P6122 to C2/c phase transition in hydrogen phase III. These experiments show the importance of a combination of ab-initio calculations and low-Z sensitive spectral probes in high-pressure science in elucidating the structural complexity of the most abundant element in our universe.

cond-mat.mtrl-sci↗

arXiv2021

Pressure driven re-entrant magnetoelectric transition in honeycomb $Fe_{4}Nb_{2}O_{9}$

A detailed high pressure investigation is carried out on $Fe_4Nb_2O_9$ using angle resolved x-ray diffraction and Raman spectroscopy measurements. We find a structural transition from the ambient trigonal phase to a monoclinic phase above 8.8 GPa. The structural transition is assumed to be driven by a large distortion of $Nb-O_6$ octahedra as seen from x-ray diffraction analysis and a large pressure dependence of $Nb-O_6$ octahedra breathing Raman mode. Anomalous behaviour of Raman modes and increase in the phonon life time at the phase transition pressure indicate a trigonal paramagnetic phase to a monoclinic antiferromagnetic state above 8.8 GPa. Decrease in the diffusive scattering rate of low frequency electronic contribution contradicts the results of decrease in intensity of high frequency electronic response and excludes the phenomenon of insulator to metal transition. Instead the enhancement of the intensity of the Raman modes till about 8.8 GPa indicate a large change in ferroelectric polarization of the sample indicating pressure induced re-entrant magentoelectric effect in $Fe_4Nb_2O_9$.

cond-mat.mtrl-sci↗

arXiv2020

Anomalous structural behavior and antiferroelectricity in BiGdO$_{3}$: Detailed temperature and high pressure study

A detailed temperature and pressure investigation on BiGdO$_{3}$ is carried out by means of dielectric constant, piezoelectric current, polarization-electric field loop, Raman scattering and x-ray diffraction measurements. Temperature dependent dielectric constant and dielectric loss show two anomalies at about 290 K (T$_r$) and 720 K (T$_C$). The later anomaly is most likely due to antiferroelectric to paraelectric transition as hinted by piezoelectric current and polarization-electric field loop measurements at room temperature, while the former anomaly suggests reorientation of polarization. Cubic to orthorhombic structural transition is observed at about 10 GPa in high pressure x-ray diffraction studies accompanied by anisotropic lattice parameter changes. An expansion about 30 % along $a$-axis and 15 % contraction along $b$-axis during the structural transition result in 9.5 % expansion in unit cell volume. This structural transition is corroborated by anomalous softening and large increase in full width half maximum (FWHM) of 640 cm$^{-1}$ Raman mode above 10 GPa. Enhancement of large structural distortion and significant volume expansion during the structural transition indicate towards an antiferroelectric to ferroelectric transition in the system.

cond-mat.mtrl-sci↗

arXiv2015

Direct Observation of Re-entrant Multiferroic CuO at High Pressures

We have carried out a detailed experimental investigation on CuO using dielectric constant, ac resistance, Raman spectroscopy and X-ray diffraction measurements at high pressures and room temperature. Both dielectric constant and dielectric loss show an anomalous peak in the pressure range 3.4-4 GPa indicating a ferroelectric transition. Raman studies show anomalous behaviour of the Ag mode with a slope change in the mode frequency and a minimum in the mode FWHM at 3.4 GPa indicating a strong spin phonon coupling along [1 0 -1] direction. A step like behaviour in the intensity of the Ag mode is observed at 3.4 GPa, indicating a change in the polarization of the mode. A maximum in the intensity of (2,0,-2)Bragg peak at 3.4 GPa points the occurrence of critical scattering due to emergence of magnetic exchange interaction. All our experimental evidences show to the presence of re-entrant type-II multiferroic behaviour in CuO at about 4 GPa.

cond-mat.mtrl-sci↗