Linear thermal expansivity and Cp measurements for LuHx and LuDx (x=0.005 and 0.053) single crystals.

Abstract

Linear thermal expansivity ({alpha}) measurements from 1 to 300 K and heat capacity ({ital C}{sub {ital p}}) measurements from 1 to 110 K are reported for single crystals of the {alpha}-Lu hexagonal alloys LuH{sub {ital x}} and LuD{sub {ital x}} ({ital x}=0.005 and 0.053). The {ital C}{sub {ital p}} data confirm and extend to 110 K earlier 1 to 20 K measurements on LuH{sub {ital x}} alloys, and show that isotope effects, if any, are small. The small {ital x} dependences of the {ital C}{sub {ital p}}{close_quote}s above 8 K can be associated with small increases in the Debye temperature {Theta}{sub 0}. This latter interpretation, which is consistent with ultrasonic and other results for these alloys, is valid only if {ital C}{sub {ital p}} is expressed as mJ/gmolK (essentially, per mole of Lu ions). Results are in agreement with the previous conclusion from more extensive polycrystalline LuH{sub {ital x}} data that the shape of the low-temperature {ital C}{sub {ital p}} vs {ital T} relation for {ital x}{le}0.015 is qualitatively different from that for {ital x}{ge}0.032. The linear thermal expansivities of the {ital c}-axis alloys, even for {ital x}=0.005, are significantly smaller (by from 3{percent} to 20{percent}) than those for themore » pure crystals, with large isotope effects and a large, nonlinear {ital x} dependence for the low-temperature expansivity data. Other types of data have shown a feature near 170 K which is associated with the completion of pairing of the hydrogens along the {ital c} axis in next-nearest-neighbor tetrahedral sites. A distinct (approximately 15 K wide) change in {alpha} which is observed near this temperature for each alloy, except for {ital a}-axis LuD{sub 0.005}, provides the most direct evidence of such a transition. The temperature of the {ital c}-axis discontinuity is slightly isotope and {ital x} dependent, and scales approximately with {ital x} [+24{percent} on warming for LuH(or D){sub 0.053}]. (Abstract Truncated)« less