Bi-apatite: Synthesis, crystal structure and low-temperature heat capacity

• Published in: The Journal of chemical thermodynamics 2018-09, Vol.124, p.74-78
• Main Authors: Bulanov, E.N., Korshak, K.S., Lelet, M.I., Knyazev, A.V., Baikie, T.
• Format: Article
• Language: English
• Subjects: Apatite; Heat capacity; In situ thermoXRD; Synthesis; Thermodynamic functions
• ISSN: 0021-9614; 1096-3626
• DOI: 10.1016/j.jct.2018.04.021

[Display omitted] •A sample of Ca8.30±0.01Bi1.70±0.01 (PO4)6O1.85±0.01 was synthesized via solid state reaction.•In-situ variable temperature X-ray diffraction measurements shows possible forming of superstructure below room-temperature.•The compound displays isotropic thermal expansion in the temperature range 173–373 K.•A thermodynamic investigation of the sample was undertaken in the temperature range 6.5–305.0 K.•An anomaly of increasing heat capacity below 8 K can be explained by the bismuth phonon contribution. In the present study, we describe a new approach to the synthesis of a Bi-containing apatite, which allows for a reduced reaction time and temperature. Using in-situ variable temperature X-ray diffraction measurements, we have refined the crystal structure of the material, which shows that its composition may be described as [Ca3.88±0.01Bi0.12±0.01]F[Ca4.42±0.01Bi1.58±0.01]T(PO4)6O1.85±0.01. The compound displays isotropic thermal expansion in the temperature range (173–373) K. A thermodynamic investigation of the sample was undertaken, which revealed the low-temperature heat capacity, Cp,mo, which was determined using adiabatic calorimetry from T = 6.4 K to 305.0 K. Smoothed Cp,mo(T) values between 6.5 K and 305.0 K are presented, along with the functions [Smo(T)-Smo(6.5)], [Hmo(T)-Hmo(6.5)], and [Φmo(T)-Φmo(6.5)]. Possible causes of the abnormal increase on the heat capacity curve in the low-temperature region are discussed.