The specific features of low-temperature thermal properties of the heterovalent (BaF₂)_0.59(TmF₃)_0.41 solid solution

ABSTRACT

The temperature dependences of the heat capacity (C_p(T)) and crystal lattice parameters (a(T) and V_u(T)) of barium difluoride, BaF₂, as well as the heterovalent (BaF₂)_0.59(TmF₃)_0.41 solid solution, in the temperature range of 2–300 K are experimentally studied. Anomalous deviations of the dependences of C_p(T), a(T) and V_u(T) of the solid solution at low temperatures (2–20 K) both in the direction of higher and lower values, due to the rearrangement of the crystal field, as well as the appearance of two-level systems in the metal sublattice of the solution, are revealed. At elevated temperatures (150–300 K), an anomalous feature of the studied characteristics with a deviation towards lower values is also observed, which we attribute to the increased influence of the anharmonicity of the vibrations of the solution lattice. It is established that the occurrence of the mechanisms of the revealed anomalies may be attributed to the structural disorder of the solid solution due to the random nature of the placement of Tm³⁺ ions when replacing Ba²⁺ ions in the metal sublattice, as well as the appearance of F⁻ ions in the interstitial spaces of the barium difluoride structure.

KEYWORDS:

• Fluorides
• heterovalent solid solution
• heat capacity
• thermal expansion
• crystal field
• structure disorder

Acknowledgments

The research was carried out with the support of the Russian Science Foundation (project No 20-72-00068).

Disclosure statement

No potential conflict of interest was reported by the author(s).

Additional information

Funding

This work was supported by Russian Science Foundation: [Grant Number 20-72-00068].