Ca. Swenson, LINEAR THERMAL EXPANSIVITY AND C-P MEASUREMENTS FOR LUHX AND LUDX (X=0.005 AND 0.053) SINGLE-CRYSTALS, Physical review. B, Condensed matter, 53(7), 1996, pp. 3680-3691
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.