Search

The ferroelectric to paraelectric phase transition in LiTaO$_3$ and in pure as well as Mg doped LiNbO$_3$ is investigated theoretically by atomistic calculations in the framework of the density functional theory, as well as experimentally by calorimetry and electrical conductivity measurements. First principles models within the stochastic self-consistent harmonic approximation (SSCHA) allow to consider anharmonic effects and thus to obtain a realistic estimate of the Curie temperature $T_C$ of both ferroelectrics. \textit{Ab initio} molecular dynamics (AIMD) calculations performed on large supercells confirm the Curie temperatures estimated with the SSCHA approach. Moreover, they also suggest that the structural phase transition is a continuous process beginning at temperatures well below $T_C$. According to AIMD, significant ionic displacements occurr already at temperatures of about 100\,K and 300\,K below $T_C$ in LiTaO$_3$ and LiNbO$_3$, respectively. To asses whether and how far the ionic displacements affect the materials properties, the AIMD results are compared with measurements of the electrical conductivity and of the heat capacity across the phase transition. Our first principles calculations moreover show that Mg ions, a frequently employed dopant, raise the Curie temperature in LiNbO$_3$.

The investigation of the structural phase transition in the vicinity of the Curie temperature $T_c$ of LiNb$_{1-x}$Ta$_x$O$_3$ crystals is motivated by the expected combination of advantageous high-temperature properties of LiNbO$_3$ and LiTaO$_3$, including high piezoelectric modules and remarkable high-temperature stability, respectively. $T_c$ marks the ultimate limit for exploiting the piezoelectric properties, however transition related structural modifications might impact this and other properties even below $T_c$. Remarkably, the phase transition from the ferroelectric to the paraelectric phase, whose temperature strongly depends on the composition $x$, shows a significant drop in the activation energy of the electrical conductivity. The magnitude, temperature dependence and underlying mechanisms of this drop are discussed from a microscopic perspective. Molecular dynamics calculations in the framework of the density functional theory show that substantial displacements of the cations occur below $T_c$ for both the end compounds LiNbO$_3$ and LiTaO$_3$, and might thus affect the electrical conductivity. Above $T_c$, the migration of lithium ions is presumably facilitated by a shortened diffusion path for the most favorable jump of the lithium ions. Electronic contributions to the conductivity, which become important above 900 K, are explained within the polaronic picture by the formation and migration of free small polarons., Comment: I submitted it to The Physica Status Solidi A: Applications and Materials Science on 12/21/2023 and it's under review. It will be updated after receiving reviews

The phase diagram of the lithium niobate and lithium tantalate solid solutions was investigated using experimental data from differential thermal analysis (DTA) and crystal growth. We used XRF analysis to determine the elemental composition of crystals. Based on the Neumann-Kopp rule, essential data of end members lithium niobate (LN) and lithium tantalate (LT) was created. The heats of fusion of end members given by DTA measurements of LN (103 kJ/mol at 1531 K) and LT (289 kJ/mol at 1913 K) were given as input parameters to generate the data. This data served as the basis for calculating a phase diagram for LN and LT solid solutions. Finally, based on the experimental data and thermodynamic solution model, the phase diagram was optimized in the Calphad Factsage module. We also generated thermodynamic parameters for Gibb's excess energy of the solid solution. A plot of segregation coefficient as a function of Ta concentration was derived from the phase diagram.

We report the growth of epitaxial LaInO3 on DyScO3(110) substrates by adsorption-controlled plasma-assisted molecular beam epitaxy (PA-MBE). The adsorption-controlled growth was monitored using line-of-sight quadrupole mass spectrometry. In a thermodynamics of MBE (TOMBE) diagram, the experimental growth window was found to be significantly narrower than the predicted one. We found the critical thickness for strain relaxation of the LaInO3 layer (lattice mismatch $\approx$ -4$\%$) to be of 1 nm using in-situ RHEED analysis. Substrate and film possess an orthorhombic crystal structure which can be approximated by a pseudo-cubic lattice. X-ray-diffraction (XRD) analysis revealed the pseudo-cube-on-pseudo-cube epitaxial relationship ofthe LaInO3 films to the DyScO3 substrates. This relation was confirmed by transmission electron microscopy (TEM), which further resolved the presence of rotational orthorhombic domains - the majority of which have coinciding c-axis with that of the substrate. Raman spectroscopy further confirmed the presence of a LaInO3 layer. Our findings open up the possibility for 2-dimensional electron gases at the MBE-grown heterointerface with BaSnO3., Comment: 9 pages, 11 figures

By differential thermal analysis a concentration field suitable for the growth of Zr, Mg codoped strontium hexagallate crystals was observed that corresponds well with experimental results from Mateika and Laurien, J. Crystal Growth 52 (1981) 566-572. It was shown that the melting point of doped crystal is ca. 60 K higher than that of undoped crystals. This higher melting points indicates hexagallate phase stabilization by Zr, Mg codoping, and increases the growth window, compared to undoped SrO-Ga$_2$O$_3$ melts., Comment: 8 pages, 4 figures, 1 table, 31 references

CuFeO$_2$ single crystals up to 50 mm in length and up to 10 mm in diameter were grown by the optical floating-zone method. Stoichiometric polycrystalline rods with a diameter of 6-12 mm were used as feed materials to produce crystals of sufficient size to be used as substrates for the growth of thin films of delafossites. For stable growth along the $c$-axis, low growth rates of 0.4 mm/h are necessary. Due to the incongruent melting behavior of CuFeO$_2$, a stable melt zone requires adjustment of the lamp power during growth. The melting of CuFeO$_2$ is not simply incongruent because the thermodynamic equilibrium includes more than two solid phases and the melt; the gas phase is also involved. The crystals were characterized by X-ray diffraction and X-ray fluorescence measurements., Comment: 17 pages, 5 figures, 1 table

The systems Nb$_2$O$_5$-Ta$_2$O$_5$ and Nb$_2$O$_5$-V$_2$O$_5$ were investigated using thermal analysis, X-ray powder diffraction and thermodynamic simulations. Solid solution formation is possible for both systems; furthermore, both contain one intermediate compound, VNb$_9$O$_{25}$ or Ta$_2$Nb$_4$O$_{15}$, respectively. Phase relationships for pure niobium(V)-oxide and tantalum(V)-oxide were studied under ambient pressure. It was found that both compounds can occur in two stable solid modifications. For niobium(V)-oxide this are the monoclinic high-temperature modification (H-Nb$_2$O$_5$) and an orthorhombic low-temperature modification (T-Nb$_2$O$_5$) and for tantalum(V)-oxide a tetragonal high-temperature form ($\alpha$-Ta$_2$O$_5$) and an orthorhombic low-temperature form ($\beta$-Ta$_2$O$_5$). Based on these results, crystal growth experiments with various compositions from both systems were carried out using the optical floating zone (OFZ) technique., Comment: 15 pages, 10 figures, 6 tables, submitted to J. Crystal Growth

Based on differential thermal analysis (DTA) and X-ray powder diffraction (XRD), a description of the system Nd$_2$O$_3$--Lu$_2$O$_3$--Sc$_2$O$_3$ was obtained by thermodynamic assessment. Four fields of primary crystallization could be identified; from melt compositions close to the Lu$_2$O$_3$--Sc$_2$O$_3$ edge the rare-earth oxide C-phase crystallizes first, which is stable down to room temperature. From Nd$_2$O$_3$ rich melts the X-phase forms, which is stable only at high temperatures. An additional field, where the alternative high-temperature phase H solidifies as primary product touches the Nd$_2$O$_3$--Lu$_2$O$_3$ edge of the concentration triangle. From melts close to the composition NdScO$_3$, the P-phase (perovskite) can be crystallized and mixed crystals with second end member NdLuO$_3$ have been grown from the melt. Crystals of this mixed perovskite were grown by the micro-pulling-down and Czochralski methods., Comment: 16 pages, 8 figures, submitted to J. Crystal Growth

When calcium titanate crystals are grown from stoichiometric melts, they crystallize in the cubic perovskite structure. Upon cooling to room temperature they undergo subsequent phase transitions to tetragonal and orthorhombic modifications. These phase transitions are disruptive and result in severely damaged crystals. This paper presents differential thermal analysis data for several prospective solvents, with the aim to identify a system offering the possibility to perform crystal growth of undistorted CaTiO$_3$ crystals by crystallizing them significantly below the melting point directly in the low temperature modification. From mixtures CaF$_2$:TiO$_2$:CaTiO$_3$ = 3:1:1 (molar ratio) the growth of undistorted, at least millimeter-sized CaTiO$_3$ crystals is possible., Comment: 12 pages, 6 figures, submitted to J. Crystal Growth

High-temperature thermal properties of three neighboring rare-earth scandates DyScO$_3$, TbScO$_3$ and GdScO$_3$ were compared to La$_{0.29}$Sr$_{0.71}$Al$_{0.65}$Ta$_{0.35}$O$_3$ (LSAT) and sapphire. To calculate thermal conductivity, heat capacity and thermal diffusivity were measured by differential scanning calorimetry and laser flash technique, respectively. DyScO$_3$ and TbScO$_3$ showed an untypical rise in the thermal conductivity above 900\,K, while for GdScO$_3$, LSAT and sapphire the expected decrease at elevated temperatures could be observed. These results lead to the proposal of a new type of heat transport by migrating ions., Comment: 15 pages, 4 figures, submitted to J. Alloys. Compd

Simultaneous differential thermal analysis (DTA) and thermogravimetric (TG) measurements with copper oxide/aluminum oxide mixtures were performed in atmospheres with varying oxygen partial pressures and with crucibles made of different materials. Only sapphire and platinum crucibles proved to be stable under conditions that are useful for the growth of CuAlO$_2$ delafossite single crystals. Then the ternary phase diagram Al$_2$O$_3$-CuO-Cu and its isopleth section Cu$_2$O-Al$_2$O$_3$ were redetermined. Millimeter sized crystals could be obtained from copper oxide melts with 1-2 mol % addition of aluminum oxide that are stable in platinum crucibles held in oxidizing atmosphere containing 15-21 % oxygen., Comment: 12 pages, 7 figures

Measurements of thermal diffusivity \k{appa} (using laser flash analysis) for temperatures from 100 {\deg}C to 1200 {\deg}C and of the inversion parameter x (using X-ray analysis) for the single crystal spinel MgGa2O4 have been performed. X-ray analysis showed a small change in x for samples annealed at 800 {\deg}C (x = 0.838) and 1200 {\deg}C (x = 0.834). Thermal diffusivity revealed a significant rise of \k{appa} for temperatures higher than 950 {\deg}C, deviating from the typical 1/T drop of \k{appa}. An additional systematic rise of \k{appa} with time is observed for temperatures between 950 {\deg}C and 1100 {\deg}C. A connection between cation disordering processes typical for spinels at elevated temperatures and the observed deviation of \k{appa} is proposed: Data indicate that cations changing their occupancy site at equilibration processes contribute significantly to \k{appa} and therefore represent a new type of heat transport., Comment: 6 pages, submitted to Crystal Research & Technology

First single crystal fibers of cerium doped strontium yttrate are fabricated using the laser-heated pedestal growth technique. Through thermodynamic equilibrium calculations and by high-temperature mass spectrometry suitable growth conditions can be determined. The atmosphere plays an important role during crystallization. It affects the composition shift, on the one hand, and the valence state of cerium, on the other hand. These properties can be explained by combining X-ray diffraction, elemental analysis, and optical spectroscopy. Crystallization in slightly reducing nitrogen atmosphere proves to be a reasonable choice, because evaporation is suppressed and trivalent cerium is stabilized. Strong green emission that depends on the oxygen fugacity during crystallization can be excited using UV light. Optical transmission of SrY2O4:Ce3+ is measured for the first time.

Ce3+ is known to show broad optical emission peaking in the green spectral range. For the stabilization of 3-valent cerium in ceramic phosphors such as calcium scandate CaSc2O4, often co-doping with sodium for charge compensation is performed (Na+, Ce3+ = 2 Ca2+). At the melting point of CaSc2O4 (ca. 2110 deg C), however, alkaline oxides evaporate completely and co-doping is thus no option for crystal growth from the melt. It is shown that even without co-doping Ce3+:CaSc2O4 crystal fibers can be grown from the melt by laser-heated pedestal growth (LHPG) in a suitable reactive atmosphere. Reactive means here that the oxygen partial pressure is a function of temperature and pO2(T) rises for this atmosphere in such a way that Ce3+ is kept stable for all T. Crystal fibers with ca. 1 mm diameter and up to 50 mm length were grown and characterized. Differential thermal analysis (DTA) was performed in the pseudo-binary system CaO-Sc2O3, and the specific heat capacity cp(T) of CaSc2O4 was measured up to 1240 K by differential scanning calorimetry (DSC). Near and beyond the melting point of calcium scandate significant evaporation of calcium tends to shift the melt composition towards the Sc2O3 side. Measurements and thermodynamic calculations reveal quantitative data on the fugacities of evaporating species., Comment: 5 pages, 4 figures, MRS Fall Boston 2013

The TiO2 rich part of the (1-x) SrO + x TiO2 phase diagram 0.5 <= x <= 1.0 was redetermined and the eutectic point between SrTiO3 and TiO2 was found at x_eut=0.7700+/-0.0001; T_eut=(1449+/-3) deg C. From TiO2 rich melt solutions, x=0.75 centimeter-sized single crystals could be grown. The best crystals with etch pit density <2 x 10^4/cm^2 were obtained for growth directions <110> and <100>. AFM investigation of the interface reveals layer-by-layer growth., Comment: 6 pages, 7 figures

Eutectic fibers consisting of an ordered arrangement of LiF fibrils inside a LiREF4 matrix (RE = Y, Gd) can be grown with the micro-pulling-down method at sufficiently large pulling rate exceeding 120 mm/h. The distance between individual fibrils could be scaled down to 1 micrometer at 300 mm/h pulling. LiF-LiYF4 has stronger tendency to form facetted eutectic colonies than LiF-LiGdF4, explained by the larger entropy of melting of the former., Comment: 6 pages, 5 figures, Talk on MRS Fall 2012 Boston

Small ruby crystals were grown by the Czochralski technique in different atmospheres and their actual chromium content was analysed by a wet chemical method. The chromium distribution coefficient k was found to be strongly dependent on oxygen partial pressure p(O2). It ranges from k=0.3 in a reducing atmosphere to k=1.2 in a slightly oxidizing atmosphere and to a good approximation k depends linearly on log(p(O2)). The experimental data are discussed on the basis of thermodynamic equilibrium calculations.

ZnO bulk crystals can be grown by several methods. 1) From the gas phase, usually by chemical vapor transport. Such CVT crystals may have high chemical purity, as the growth is performed without contact to foreign material. The crystallographic quality is often very high (free growth). 2) From melt fluxes such as alkaline hydroxides or other oxides (MoO3, V2O5, P2O5, PbO) and salts (PbCl2, PbF2). Melt fluxes offer the possibility to grow bulk ZnO under mild conditions (<1000 deg. C, atmospheric pressure), but the crystals always contain traces of solvent. The limited purity is a severe drawback, especially for electronic applications. 3) From hydrothermal fluxes, usually alkaline (KOH, LiOH) aqueous solutions beyond the critical point. Due to the amphoteric character of ZnO, the supercritical bases can dissolve it up to several per cent of mass. The technical requirements for this growth technology are generally hard, but this did not hinder its development as the basic technique for the growth of {\alpha}-quartz, and meanwhile also of zinc oxide, during the last decades. 4) From pure melts, which is the preferred technology for numerous substances applied whenever possible, e.g. for the growth of silicon, gallium arsenide, sapphire, YAG. The benefits of melt growth are (i) the high growth rate and (ii) the absence of solvent related impurities. In the case of ZnO, however, it is difficult to find container materials that are compatible from the thermal (fusion point Tf = 1975 deg. C) and chemical (required oxygen partial pressure) point of view. Either cold crucible (skull melting) or Bridgman (with reactive atmosphere) techniques were shown to overcome the problems that are inherent to melt growth. Reactive atmospheres allow to grow not only bulk ZnO single crystals, but also other TCOs such as {\beta}-Ga2O3 and In2O3., Comment: 10 pages, 7 figures, talk on MRS Fall 2011 Boston

The V2O5-MoO3 mixtures offer a whole range of materials where properties can be adjusted by simple modification of experimental parameters, which may be utilized in manufacturing of metamaterials with on-demand properties. The V2O5-MoO3 system contains an intermediate phase V9Mo6O40, with a small fraction of V4+ instead of V5+. Consequently, this system should rather be considered as pseudobinary. The V4+ content depends on the oxygen partial pressure in the atmosphere. Thus, by changing the oxygen partial pressure one can tailor the electric properties of the system, and by changing the supercooling, the morphologic structure of crystallized bodies as well. For better understanding of this system differential thermal analysis and thermodynamic modeling was performed. Fibers of eutectic composition between V9Mo6O40 and MoO3 were grown by the micro-pulling-down technique. X-ray diffraction confirmed the existence of the V9Mo6O40 intermediate phase., Comment: 8 pages, 6 figures, MRS Fall 2009 Boston

Bulk crystals of zinc oxide can be grown from the melt by a Bridgman technique under pressure. This new technology using an iridium crucible shows the potential to yield large single crystals of good crystalline perfection. Crystals with diameters up to 33 mm and a length of up to 50 mm have been demonstrated. The impurity content can be strongly reduced by using the crucibles repeatedly., Comment: 6 pages, 2 figures, 1 table, talk on MRS Fall 2009 Boston, MA

The scheelite type laser crystals LiREF4 melt congruently only for RE being one of the elements Er, Tm, Yb, Lu, or possibly Y, respectively. For RE = Eu, Gd, Tb, Dy, or Ho the corresponding scheelites undergo a peritectic melting under the formation of the corresponding rare earth fluoride. The melting behavior of LiREF4 mixed crystals with two or more RE is not yet known well. If RE is a mixture of Gd and Lu, Gd rich solid solutions melt peritectically under formation of (Gd,Lu)F3 and Lu rich solid solutions melt directly without formation of other solid phases., Comment: 6 pages, 4 figures, MRS Fall 2008 Boston

DTA measurements with mixtures of aluminum oxide and lutetium oxide around the 1:1 perovskite composition were performed up to 1970 deg. C. A peak with onset 1901 deg C was due to the melting of the eutectic Lu4Al2O9 (monoclinic phase) and LuAlO3 (perovskite). Neither peritectic melting of the perovskite nor its decomposition in the solid phase could be resolved experimentally. The maximum of the eutectic peak size is near x=0.44, on the Lu-rich side of the perovskite, which is consistent with the conclusion that LuAlO3 melts peritectically at ca. 1907 deg. C as proposed by Wu, Pelton, J. Alloys Compd. 179 (1992) 259. Thermodynamic equilibrium calculations reveal, that under strongly reducing conditions (oxygen partial pressure <10^{-13} bar) aluminum(III) oxide can be reduced to suboxides or even Al metal. It is shown that under such conditions a new phase field with liquid Al can appear., Comment: 6 pages, 4 figures, 1 table, submitted to J. Crystal Growth