We present a systematic investigation of the low-temperature specific heat of single-crystalline NaCl-type UX compounds (X=N, P, As, Sb, S, Se, Te) at temperatures between 0.12 and 12 K. It is primarily intended to determine the low-temperature electronic specific heats ${C}_{e}$=\ensuremath{\gamma}T of these materials. From our experiments, \ensuremath{\gamma} increases from UN (25.8 mJ/mole ${\mathrm{K}}^{2}$) to UAs by a factor of about 2, but is an order of magnitude smaller in USb. For the chalcogenides, a decreasing electronic specific heat with increasing anion size is observed. The measurements at the lowest temperatures reveal the onset of a nuclear Schottky anomaly, from which we derive magnetic hyperfine fields and obtain good agreement with published values from M\"ossbauer experiments for UP and USb. Except for USb, the nuclear heat capacity is negligible at temperatures above 1.5 K.

Low-temperature specific heat of uranium monopnictides and monochalcogenides.

The specific heats of UAs and the isostructural nonmagnetic homolog ThAs have been measured in the temperature range 5--300 K. While the latter compound displays a regular smooth curve C/sub p/(T), UAs shows tow sharp anomalies. The first anomaly, around 64 K, may be ascribed to the magnetic transition from type IA to type I antiferromagnetic structure; the second anomaly, at 122.8 K, corresponds to the Neel temperature. An analysis of the experimental curve C/sub p/(T) for UAs has been carried out by several different methods to get the magnetic contribution to the specit with the best psosible accuracy. The resulting magnetic entgropy depends on the methods on the method and its maximum value at 250 K is 0.8R ln 4, assuming a high-temperature value of the electronic heat capacity coefficient approx. =33 mJ/K/sup 2/ mole. No anomaly at 41 K was observed whatever thermal treatment was used to prepare the UAS samples.

The heat capacity of uranium monoarsenide

The properties of point defects responsible for non-stoichiometry and thermal disorder in actinide monocarbides and dioxides are reviewed: atomic structure, formation and interaction energies, influence on electronic properties, migration properties. Recent experimental and theoretical evidence for a lattice deformation around carbon vacancies in actinide monocarbides, and its influence on the stability of the phases, are discussed in detail.

Defect structures in actinide compounds

The microscopic magnetic properties of UP with the cubic NaCl-type structure have been investigated by 31 P NMR in the magnetically ordered state. 1/ T 1 is extraordinarily large in the single- Q type-I phase ($T_{0}\cong 22.5$ K$<t<t_{\rm N}\cong 122$ K), and shows about two orders of magnitude abrupt decrease through the single- Q ↔double- Q phase transition at T 0 . Below about 4 K in the double- Q type-I phase ( T < T 0 ), 1/ T 1 shows a Korringa-like temperature-dependence, which corresponds qualitatively to a 5 f -derived γ T term in the specific heat. We discuss this behavior as a clear indication of the existence of 5 f -derived very-low-energy magnetic excitations even in the ordered state, and an incipient heavy-electron character of the compound is concluded. We also present a simple qualitative model of a system of exchange-coupled Kondo impurities under internal exchange (or molecular) field in order to discuss the origin of these excitations.

Low-Energy Magnetic Excitations in the Antiferromagnetically Ordered State of an Incipient Heavy-Electron Compound UP as Studied by 31P NMR

The specific heat of ThC0,75, ThC0,6N0,4 and ThP samples has been measured in the range 2-15 K. From the results, the Debye temperature θD and the density of states at the Fermi level NF of the three compounds have been calculated. NF increases with vacancy content in ThC1-x and is minimal for ThC0,6N0,4 compared to ThC and ThN.

https://hal.archives-ouvertes.fr/jpa-00218841/document

J. L. Boutard

Papers

Low temperature specific heat of rocksalt thorium compounds