LINEAR THERMAL EXPANSIVITY AND C-P MEASUREMENTS FOR LUHX AND LUDX (X=0.005 AND 0.053) SINGLE-CRYSTALS

Linear thermal expansivity (alpha) measurements from 1 to 300 K and he at capacity (C-p) measurements from 1 to 110 K are reported for single crystals of the alpha-Lu hexagonal alloys LuHx and LuDx (x=0.005 and 0.053). The C-p data confirm and extend to 110 K earlier 1 to 20 K mea surements on LuHx alloys, and, in addition, show that isotope effects, if any, are small. The small x dependences of the C(p)s above 8 K can be associated with small increases in the Debye temperature Theta(0). This latter interpretation, which is consistent with ultrasonic and o ther results for these alloys, is valid only if C, is expressed as mJ/ g mol K (essentially, per mole of Lu ions). The present results are in agreement with the previous conclusion from more extensive polycrysta lline LuHx data that the shape of the low-temperature C-p vs T relatio n for x less than or equal to 0.015 is qualitatively different from th at for x greater than or equal to 0.032. The linear thermal expansivit ies of the c-axis alloys, even for x=0.005, are significantly smaller (by from 3% to 20%) than those for the pure crystals, with large isoto pe effects and a large, nonlinear 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 a long the c axis in next-nearest-neighbor tetrahedral sites. A distinct (approximately 15 K wide) change in cu which is observed near this te mperature for each alloy, except for a-axis LuD0.005, provides the mos t direct evidence of such a transition. The temperature of the c-axis discontinuity is slightly isotope and x dependent, and scales approxim ately with x [+24% on warming for LuH(or D)(0.053)]. The LuH0.053 a-ax is alpha's show a transition at the same temperature but of opposite s ign (-15% on warming). A large relative decrease in the expansivities of each of the alloys between T=0 and the transition can be ascribed t o a more rapid disappearance of the spin-fluctuation and electron-phon on enhancement terms for the alloys than for the pure metal. The large differences in the isotope and x dependences of the alpha vs T relati ons support the postulate that the state of these alloys is quite diff erent for x less than or equal to 0.015 and x greater than or equal to 0.032.