arXiv subjects

D. Klimm

Publications and source records attributed to D. Klimm.

At least 19 recordsLinked to original sources

arXiv2019

Single crystal growth and characterization of Ba$_2$ScNbO$_6$ -- a novel substrate for BaSnO$_3$ films

Large single crystals of the double-perovskite Ba$_2$ScNbO$_6$ were grown from the melt for the first time. With a lattice parameter at room temperature of 4.11672(1) Å, this cubic double-perovskite has an excellent lattice match to BaSnO$_3$, PbZr$_{0.9}$Ti$_{0.1}$O$_3$, LaInO$_3$, BiScO$_3$, and other perovskites of contemporary interest. Differential thermal analysis showed that Ba$_2$ScNbO$_6$ melts at 2165$\pm$30°C in an inert atmosphere. Competitive grain growth was visualized by energy dispersive Laue mapping. X-ray diffraction rocking curve measurements revealed full width at half maximum values between 21 and 33 arcsec for 002 and 004 reflections. The crystals were sufficiently large to yield (100)-oriented single-crystal substrates with surface areas as large as 10 x 10 mm$^2$.

cond-mat.mtrl-sci↗

arXiv2017

Crystal growth and characterization of the pyrochlore Tb$_2$Ti$_2$O$_7$

Terbium titanate (Tb$_2$Ti$_2$O$_7$) is a spin-ice material with remarkable magneto-optical properties. It has a high Verdet constant and is a promising substrate crystal for the epitaxy of quantum materials with the pyrochlore structure. Large single crystals with adequate quality of Tb$_2$Ti$_2$O$_7$ or any pyrochlore are not available so far. Here we report the growth of high-quality bulk crystals using the Czochralski method to pull crystals from the melt. Prior work using the automated Czochralski method has suffered from growth instabilities like diameter fluctuation, foot formation and subsequent spiraling shortly after the seeding stage. In this study, the volumes of the crystals were strongly increased to several cubic centimeters by means of manual growth control, leading to crystal diameters up to 40 mm and crystal lengths up to 10 mm. Rocking curve measurements revealed full width at half maximum values between 28 and 40" for 222 reflections. The specific heat capacity c$_p$ was measured between room temperature and 1573 K by dynamic differential scanning calorimetry and shows the typical slow parabolic rise. In contrast, the thermal conductivity κ(T) shows a minimum near 700 K and increases at higher temperature T. Optical spectroscopy was performed at room temperature from the ultraviolet to the near infrared region, and additionally in the near infrared region up to 1623 K. The optical transmission properties and the crystal color are interpreted to be influenced by partial oxidation of Tb$^{3+}$ to Tb$^{4+}$.

cond-mat.mtrl-sci↗

arXiv2014

Electronic Materials with Wide Band Gap: Recent Developments

The development of semiconductor electronics is shortly reviewed, beginning with the development of germanium devices (band gap $E_g=0.66$ eV) after world war II. Quickly a tendency to alternative materials with wider band gap became apparent, starting with silicon ($E_g=1.12$ eV). This improved the signal/noise ratio for classical electronic applications. Both semiconductors have tetrahedral coordination, and by isoelectronic alternative replacement of Ge or Si with carbon or several anions and cations other semiconductors with wider $E_g$ are obtained, that are transparent for visible light and belong to the group of wide band gap semiconductors. Nowadays some nitrides, especially GaN and AlN, are the most important materials for optical emission in the ultraviolet and blue spectral region. Oxide crystals, such as ZnO and $β$-Ga$_2$O$_3$, offer similarly good electronic properties but suffer still from significant difficulties in obtaining stable and technically sufficient $p$-type conductivity.

cond-mat.mtrl-sci↗

arXiv2013

Growth and Investigation of Nd_{1-x}Sm_{x}ScO_{3} and Sm_{1-x}Gd_{x}ScO_{3} Solid-Solution Single Crystals

The pseudo-cubic lattice parameters of rare-earth (RE) scandate, REScO3, single crystals grown by the Czochralski technique with RE=Dy to Pr lie between about 3.95 and 4.02 Angstrom. These crystals are the only available perovskite substrates in this lattice constant range that can withstand virtually any thin film growth conditions. Two members of this series, PmScO3 and EuScO3, are, however, not suitable for substrate applications. Because the pseudo-cubic lattice parameters between neighbouring REScO3 compounds decrease with rising atomic number of the RE in about 0.01 Angstrom steps, the unsuitability of PmScO3 (radioactivity) and EuScO3 (incompatibility with Si) causes an interruption in this lattice spacing sequence. To replace them, solid solutions of their adjacent rare-earth scandates, i.e., (Nd0.5Sm0.5)ScO3 and (Sm0.5Gd0.5)ScO3, were grown by the Czochralski method. Their average pseudo-cubic lattice parameters of 3.9979 Angstrom and 3.9784 Angstrom are very close to those of PmScO3 and EuScO3, respectively, and they show very low segregation. These qualities make these solid solutions excellent substitutes for PmScO3 and EuScO3.

cond-mat.mtrl-sci↗

arXiv2012

Laser-heated pedestal growth of cerium doped calcium scandate crystal fibers

Ce3+ doped oxide materials are promising for optical emission in the green spectral range. The growth of CaSc2O4:Ce3+ single crystals is reported here for the first time. Laser heated pedestal growth (LHPG) proved to be suitable for this refractive material, if performed in nitrogen of 99.999% purity. If the oxygen content of the growth atmosphere is substantially larger, Ce4+ is formed, which shows no useful optical emission. If the oxygen content is substantially lower, severe evaporation of calcium impedes stable crystal growth. Thermodynamic equilibrium calculations allowed to describe evaporation of species and cerium dopant charging under different growth conditions. The evaporation could be investigated by quadrupole mass spectrometry of emanating gases and by chemical analysis of fibers with ICP-OES. The congruent melting point was confirmed by DTA at 2110 degrees centigrade. Photoluminescence spectrometry of fibers revealed the dependence of optical emission in the green spectral range on growth conditions.

cond-mat.mtrl-sci↗

arXiv2012

Experimental evaluation and thermodynamic assessment of the LiF-LuF3 phase diagram

The phase diagram of the system LiF-LuF3 has been revised using thermal analysis. Specific heat capacity and enthalpy of phase transition and fusion were measured by differential scanning calorimetry for all compounds belonging to the system. A thermodynamic optimization of the LiF-LuF3 phase diagram was performed by fitting the Gibbs energy functions to experimental data that were taken from the literature or measured in this work. Excess energy terms, which describe the effect of interaction between the two fluoride compounds in the liquid solution, were expressed by the Redlich-Kister polynomial function. The theoretical phase diagram assessed was in suitable agreement with the re-evaluated experimental data.

cond-mat.mtrl-sci↗

arXiv2012

Crystal growth of LiGd1-xLuxF4 solid solutions by zone melting technique

Mixtures (1 - x) LiGdF4 + x LiLuF4 (0.50? \leq x \leq 0.75) were melted and submitted to one zone melting cycle under a reactive HF atmosphere, aiming the study of the crystal growth of LiGd1-xLuxF4 solid solutions. Phase identification and compositional studies of the ingots were performed by scanning electron microscopy, energy dispersive X-ray spectrometry, and X-ray powder diffraction. Transparent regions of the bar related with the formation of LiGd1-xLuxF4 solid solutions were found to be enlarged proportionally to the initial amount of Lu in the mixtures. Gd segregates to the end of the ingot.

cond-mat.mtrl-sci↗

arXiv2012

Thermodynamic modeling of the LiF-YF3 phase diagram

A thermodynamic optimization of the LiF-YF3 binary phase diagram was performed by fitting the Gibbs energy functions to experimental data that were taken from the literature, as well as from own thermoanalytic measurements (DTA and DSC) on HF-treated samples. The Gibbs energy functions for the end member compounds were taken from the literature. Excess energy terms, which describe the effect of interaction between the two fluoride compounds in the liquid phase, were expressed by the Redlich-Kister polynomial function. The calculated phase diagram and thermodynamic properties for the unique formed compound, LiYF4, are in reasonable agreement with the experimental data.

cond-mat.mtrl-sci↗

arXiv2012

Thermal Analysis and Phase Diagram of the LiF-BiF3 System

Differential thermal analysis up to complete melting was performed for the complete pseudobinary system LiF-BiF3. Melts with high bismuth fluoride concentration show severe evaporation, nevertheless even pure BiF3 could be molten at 655 deg. C. The system contains one intermediate compound BiLiF4 which melts by peritectic decomposition under the formation of LiF at 415 deg. C. The eutectic between BiLiF4 and BiF3 melts at 415 deg. C. By thermodynamic assessment the following parameters were found for BiLiF4: ΔH=-1514900 J/mol, S=158.5 J/mol K, c_p=166.173 -0.01072 T (in J/mol K). The melt has negative excess Gibbs free energy.

cond-mat.mtrl-sci↗

arXiv2011

Re-determination of the Pseudobinary System Li2O-MoO3

The quasi-binary phase diagram lithium oxide - molybdenum(VI) oxide was investigated by differential scanning calorimetry and X-ray diffraction. The four intermediate phases Li4MoO5, Li2MoO4, Li4Mo5O17, and Li2Mo4O13 show incongruent melting. The system has one eutectic point at 50.5 mol% MoO3 and 49.5 mol% LiO0.5 with a eutectic temperature of 524.6 deg C. At this point the melt is in equilibrium with Li2MoO4 and Li4Mo5O17.

cond-mat.mtrl-sci↗

arXiv2011

The investigation of YAlO3-NdAlO3 system, synthesis and characterization

The binary phase diagram of the YAlO3 (YAP) - NdAlO3 (NAP) system was determined by differential thermal analysis (DTA) and X-ray powder diffraction (XRD) measurements. High purity nanocrystalline powders and small single crystals of Y_{1-x}Nd_{x}AlO_3 (0 \leq x \leq 1) have been produced successfully by modified sol-gel (Pechini) and micro-pulling-down methods, respectively. Both end members show high mutual solubility >25% in the solid phase, with a miscibility gap for intermediate compositions. A solid solution with x \approx 0.2 melts azeotropic ca. 20 degrees below pure YAP. Such crystals can be grown from the melt without segregation. The narrow solid/liquid region near the azeotrope point could be measured with a "cycling" DTA measurement technique.

cond-mat.mtrl-sci↗

arXiv2009

Effect of Cd2+ on the Growth and Thermal Properties of K2SO4 crystal

Single crystals of pure and Cd2+ doped potassium sulfate were grown from aqueous solutions by the slow evaporation technique. From nutrient solutions with a CdSO4 concentration of 4wt.% crystals containing 0.014wt.% dopant concentration could be obtained. The X-ray diffraction patterns of powdered crystals confirmed their crystal structures for both cases. Thermal analysis of pure crystals shows that the alpha-beta phase transformation peak around 580 deg C is superimposed with spurious effects, while for Cd2+ doped crystals this is not the case. The thermal hysteresis of the phase transition is 8 K for undoped K2SO4 and is reduced to 3.5 K for K2SO4:Cd2+. Compared to undoped crystals, the optical transmittance of Cd2+ doped crystals is higher.

cond-mat.mtrl-sci↗

arXiv2008

Growth of Oxide Compounds under Dynamic Atmosphere Composition

Commercially available gases contain residual impurities leading to a background oxygen partial pressure of typically several 10^{-6} bar, independent of temperature. This oxygen partial pressure is inappropriate for the growth of some single crystals where the desired oxidation state possesses a narrow stability field. Equilibrium thermodynamic calculations allow the determination of dynamic atmosphere compositions yielding such self adjusting and temperature dependent oxygen partial pressures, that crystals like ZnO, Ga2O3, or Fe{1-x}O can be grown from the melt.

cond-mat.mtrl-sci↗

arXiv2008

On the solubility of Nd3+ in Y3Al5O12

Neodymium doped yttrium aluminum garnet powders (Nd:YAG or (Y{1-x}Nd{x})3Al5O12, x=0.15, 0.25, or 0.30, respectively) were prepared by a sol-gel technique. By DTA measurements up to 2000 deg C eutectic and liquidus temperatures could be determined. Exothermal peaks in the second and subsequent DTA heating runs indicate that the crystallized DTA samples are not in equilibrium. The section Y3Al5O12-Nd3Al5O12 of the concentration triangle Al2O3-Nd2O3-Y2O3 is proposed on the basis of thermodynamic calculations that allows to explain the experimental results by the balance of metastable phase states in the previously crystallized DTA samples.

cond-mat.mtrl-sci↗

arXiv2008

The growth of ZnO crystals from the melt

The peculiar properties of zinc oxide (ZnO) make this material interesting for very different applications like light emitting diodes, lasers, and piezoelectric transducers. Most of these applications are based on epitaxial ZnO layers grown on suitable substrates, preferably bulk ZnO. Unfortunately the thermochemical properties of ZnO make the growth of single crystals difficult: the triple point 1975 deg C., 1.06 bar and the high oxygen fugacity at the melting point p_O2 = 0.35 bar lead to the prevailing opinion that ZnO crystals for technical applications can only be grown either by a hydrothermal method or from "cold crucibles" of solid ZnO. Both methods are known to have significant drawbacks. Our thermodynamic calculations and crystal growth experiments show, that in contrast to widely accepted assumptions, ZnO can be molten in metallic crucibles, if an atmosphere with "self adjusting" p_O2 is used. This new result is believed to offer new perspectives for ZnO crystal growth by established standard techniques like the Bridgman method.

cond-mat.mtrl-sci↗

arXiv2008

The Phase Diagram GdF3-LuF3

The phase diagram gadolinium fluoride - lutetium fluoride was determined by differential scanning calorimetry (DSC) and X-ray powder diffraction analysis. Both pure components undergo a reversible first order transformation to a high temperature phase. The mutual solubility of both components is unlimited in the orthorhombic room temperature phase. The maximum solubility of Lu in the high temperature phase of GdF3 (tysonite type) is about 20% and the maximum solubility of Gd in LuF3 ($α$-YF3 type) is about 40%. Intermediate compositions of the low temperature phase decompose upon heating in a peritectoid reaction to a mixture of both high temperature phases.

cond-mat.mtrl-sci↗

arXiv2008

On the Crystallization of Terbium Aluminium Garnet

Attempts to grow terbium aluminium garnet (Tb3Al5O12, TAG) by the Czochralski method lead to crystals of millimeter scale. Larger crystals could not be obtained. DTA measurements within the binary system showed that TAG melts incongruently at 1840 deg. C. The perovskite (TbAlO3, TAP) with a congruent melting point of 1930 deg. C is the most stable phase in this system. The region for primary crystallization of TAP covers the chemical composition of TAG and suppresses the primary crystallization of the terbium aluminium garnet.

cond-mat.mtrl-sci↗

arXiv2008

The solid state phase transformation of potassium sulfate

Potassium sulfate single crystals that are grown from aqueous solutions lose upon the first heating up to 1% of mass that is assumed to be water. This mass loss occurs in the vicinity of the PT from orthorhombic to hexagonal K2SO4. Only in the first heating run of K2SO4 that has not yet released water, pretransitional thermal effects can be observed in the DTA curve. If K2SO4 crystals are grown from solutions containing 4 wt.% Cd, Cu, or Fe, only Cu or Fe can be incorporated significantly with concentrations of several 0.1%. The phase transformation temperature measured for such solid solutions depends on the heating rate. For pure K2SO4, the phase transformation temperature is independent on heating rate 581.3 deg. C and the enthalpy of transformation is (5.8+/-0.2) kJ/mol.

cond-mat.mtrl-sci↗