V. V. Novikov

Low-temperature heat capacity of rare-earth tetraborides

The paper reports an experimental study of the temperature dependences of the heat capacity of tetraborides of lanthanum, dysprosium, holmium, and lutetium in the range 2–300 K. The electronic, lattice, and magnetic contributions to the total heat capacity of the borides have been identified and analyzed, and the Schottky contribution has been calculated. The ground-state multiplicities of dysprosium and holmium ions in the tetraborides have been determined.

Spin glass and glass-like lattice behaviour in HoB66at low temperatures

The low-temperature specific heat, magnetic susceptibility, and thermal expansion are studied experimentally in the fcc boride HoB66 and compared to similar quantities in non-magnetic LuB66. The anomalous behaviour observed in HoB66 is explained within a glass-like picture together with magnetic subsystem ordering processes. The character of the observed anomalies suggests the existence of a spin-glass phase transition in HoB66 below a characteristic temperature of T s  ≈ 0.98 K.

Anomalies in thermal expansion of rare-earth diborides in the temperature range of magnetic phase transformations

The variations with temperature of the lattice parameters a(T) and c(T) and the thermal expansion coefficients of rare-earth diborides RB2(R=Tb, Dy, Ho, Er, Tm, Lu) have been investigated in the temperature range 4.2—300 K. It has been revealed that the dependences a(T) and c(T) exhibit an anomalous behavior in the vicinity of the ferromagnetic transformation temperatures Tc which correlates with the temperature dependences of the heat capacity. It has been established that the magnetic contributions to the thermal expansion of the diborides Δam(T) increase with increasing temperature and tend to constant values at T≫Tc and that the magnetic components Δcm(T) decrease for all the diborides studied, also tending to constant values at elevated temperatures. The exchange integrals Ya and Yc for the paramagnetic metal ions in the rare-earth diborides are determined from the dependences Δam(T) and Δcm(T).

Negative thermal expansion and anomalies of heat capacity of LuB50 at low temperatures

Heat capacity and thermal expansion of LuB50 boride were experimentally studied in the 2-300 K temperature range. The data reveal an anomalous contribution to the heat capacity at low temperatures. The value of this contribution is proportional to the first degree of temperature. It was identified that this anomaly in heat capacity is caused by the effect of disorder in the LuB50 crystalline structure and it can be described in the soft atomic potential model (SAP). The parameters of the approximation were determined. The temperature dependence of LuB50 heat capacity in the whole temperature range was approximated by the sum of SAP contribution, Debye and two Einstein components. The parameters of SAP contribution for LuB50 were compared to the corresponding values for LuB66, which was studied earlier. Negative thermal expansion at low temperatures was experimentally observed for LuB50. The analysis of the experimental temperature dependence for the Gruneisen parameter of LuB50 suggested that the low-frequency oscillations, described in SAP mode, are responsible for the negative thermal expansion. Thus, the glasslike character of the behavior of LuB50 thermal characteristics at low temperatures was confirmed.

Anomalies in thermal expansion and heat capacity of TmB50 at low temperatures: magnetic phase transition and crystal electric field effect

We experimentally study the heat capacity and thermal expansion of thulium boride (TmB50) at temperatures of 2-300 K. The wide temperature range (2-180 K) of boride negative expansion was revealed. We found the anomalies in C(T) heat capacity temperature dependence, attributed to the Schottky contribution (i.e. the influence of the crystal electric field: CEF), as well as the magnetic phase transition. CEF-splitting of the f-levels of the Tm3+ ion was described by the Schottky function of heat capacity with a quasi-quartet in the ground state. Excited multiplets are separated from the ground state by energy gaps δ1 = 100 K, and δ2 ≈ 350 K. The heat capacity maximum at Tmax ≈ 2.4 K may be attributed to the possible magnetic transition in TmB50. Other possible causes of the low-temperature maximum of C(T) dependence are the nonspherical surroundings of rare earth atoms due to the boron atoms in the crystal lattice of the boride and the emergence of two-level systems, as well as the splitting of the ground multiplet due to local magnetic fields of the neighboring ions of thulium. Anomalies in heat capacity are mapped with the thermal expansion features of boride. It is found that the TmB50 thermal expansion characteristic features are due to the influence of the CEF, as well as the asymmetry of the spatial arrangement of boron atoms around the rare earth atoms in the crystal lattice of RB50. The Grüneisen parameters, corresponding to the excitation of different multiplets of CEF-splitting, were determined. A satisfactory accordance between the experimental and estimated temperature dependencies of the boride thermal expansion coefficient was achieved.

Thermal Expansion of Dysprosium Tetraboride

The temperature variations of the interplanar spacings a(T) and c(T) in the crystal lattice of dysprosium tetraboride have been studied using X-ray diffraction in the temperature range 5–300 K. Anomalous variations of a(T) and c(T) in the temperature range of magnetic transformations, anisotropy of the thermal expansion of DyB4, and the monoclinic distortion of the crystal structure at low temperatures have been revealed. The magnitudes of the spontaneous magnetostriction, the thermal expansion coefficients αa and αc, and the exchange integrals Ya and Yc have been determined.

Specific features of phonon subsystems in diborides of rare-earth elements

The temperature dependences of the lattice heat capacity of yttrium, terbium, erbium, and lutetium diborides have been studied using Montroll’s method of moments. The characteristic temperatures of the diborides are determined at absolute zero temperature and at the temperature tending to infinity. The geometric mean frequencies of phonon spectra have been calculated. The influence of the rare-earth ion mass and the lanthanide contraction phenomenon on the lattice dynamics has been analyzed for the rare-earth diborides.

Thermal expansion and lattice dynamics of RB66 compounds at low temperatures

Thermal characteristics of the phonon and magnon subsystems of icosahedral borides RB66 (R = Gd, Tb, Dy, Ho, Eu, or Lu) have been studied based on the obtained experimental data on the thermal expansion of the borides and the earlier results on their heat capacity in the range of 2–300 K. The contribution to the expansion of borides containing paramagnetic R3+ ions, which is characteristic of transition to the spin-glass state, has been revealed. The phonon spectrum moments of RB66 compounds and the Grüneisen parameters have been calculated.

Heat capacity and characteristic thermodynamic functions of dysprosium boride DyB62 in the temperature range of 2 to 300 K

Heat capacity at constant pressure Cp(T) of a dysprosium boride DyB62 single crystal obtained by zone melting was studied experimentally in the temperature range of 2 to 300 K. Abnormally high values of dysprosium boride heat capacity were revealed in the range of 2–20 K, due to the magnetic contribution and the effect of disorder in the boride lattice. Temperature changes in DyB62 enthalpy, entropy, Gibbs energy, and standard values of these thermodynamic functions were calculated.

Thermal expansion and mean-square displacements of metal and boron atoms in dysprosium diboride DyB2

Dysprosium diboride is investigated by x-ray diffraction in the temperature range 4.2–300 K. The data obtained are used to determine the (301) and (104) interplanar distances and the corresponding intensities of the x-ray reflections. The calculated linear and bulk thermal expansion coefficients of the DyB2 compound are characterized by a pronounced anomaly in the temperature range corresponding to the magnetic transformation. The estimates of the mean-square displacements for the Dy and B atoms (calculated from the reflection intensities) are satisfactorily described by the Debye dependences with the characteristic temperatures θDy = 210 K and θB = 800 K.