@phdthesis{Ganie2025, author = {Ganie, Umar Bashir}, title = {Thermal analysis of lithium niobate tantalate bulk mixed crystals}, doi = {10.26127/BTUOpen-6981}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:co1-opus4-69813}, school = {BTU Cottbus - Senftenberg}, year = {2025}, abstract = {This study comprehensively investigated the thermal properties and phase behavior of lithium niobate-tantalate (𝐿𝑖𝑁𝑏π‘₯π‘‡π‘Ž1-π‘₯𝑂3, LNT) solid solutions. The research used experimental techniques to explore the variation of the ferroelectric Curie temperature (Tc) as a function of composition. LNT single crystals were grown using the Czochralski method. The elemental composition of niobium (Nb) and tantalum (Ta) in these crystals was determined using X-ray fluorescence (XRF) analysis. Small, compositionally homogeneous samples were then selected for differential scanning calorimetry (DSC) measurements to determine specific heat capacities (Cp). The DSC measurements revealed a linear decrease in Tc with increasing Ta concentration in the LNT solid solution crystals. Additionally, the ferroelectric transition width was observed to be narrower in mixed crystals compared to pure LT. Differential Thermal Analysis (DTA) and crystal growth experiments were performed further to understand the phase behavior of LNT solid solutions. The heats of fusion for the end members, LN and LT, were measured using DTA, yielding values of 103 kJ/mol at 1531 K for LN and 289 kJ/mol at 1913 K for LT. These values were used as input parameters in a thermodynamic solution model implemented in the Factsage software. The solution model enabled the calculation of a phase diagram for LNT solid solutions, which was further optimized in the Calphad Factsage. Thermodynamic parameters for the Gibbs free energy of mixing of the solid solution were also generated. The resulting phase diagram showed good agreement with the experimental data. Additionally, the temperature-dependent thermal conductivity of pure LN, LT, and LNT solid solutions and selected doped LN and LT crystals (Mg, Zn) was investigated. Measurements were conducted across a temperature range from 300 K to 1300 K. The findings indicated that thermal conductivity increases with temperature, especially above 800 K, with a more pronounced effect in tantalum-rich solutions. The interplay of the Nb/Ta ratio and doping effects was particularly significant at high temperatures. These insights into the thermal conductivity of LNT and doped LN and LT crystals are crucial for optimizing growth conditions. Understanding thermal conductivity helps ensure homogeneous crystallization during the growth process, which remains a challenge in the production of LNT single crystals.}, subject = {Thermal conductivity; Lithium niobate tantalate; Mixed crystals; Crystal growth; Thermal analysis; Phase diagram; Mischkristalle; Kristallz{\"u}chtung; Thermische Analyse; Phasendiagramm; W{\"a}rmeleitf{\"a}higkeit; Tantalate; Lithium; Niob; Mischkristall; Phasendiagramm; Temperaturabh{\"a}ngigkeit; Phasengleichgewicht}, language = {en} }