Structural and Magnetic Phase Transitions in CeCu${}_{6-x}{T}_x$ ($T$ = Ag, Pd)

Abstract

The structural and the magnetic properties of CeCu${}_{6-x}$Ag${}_x$ (0 $\le$ $x$ $\le$ 0.85) and CeCu${}_{6-x}$Pd${}_x$ (0 $\le$ $x$ $\le$ 0.4) have been studied using neutron diffraction, resonant ultrasound spectroscopy (RUS), heat capacity, x-ray diffraction measurements and first principles calculations. The structural and magnetic phase diagrams of CeCu${}_{6-x}$Ag${}_x$ and CeCu${}_{6-x}$Pd${}_x$ as a function of Ag/Pd composition are reported. The end member, CeCu${}_6$, undergoes a structural phase transition from an orthorhombic ($Pnma$) to a monoclinic ($P{2}_1/c$) phase at 240 K. In CeCu${}_{6-x}$Ag${}_x$, the structural phase transition temperature ($T_s$) decreases linearly with Ag concentration and extrapolates to zero at $x_S$ $\approx$ 0.1. The structural transition in CeCu${}_{6-x}$Pd${}_x$ remains unperturbed with Pd substitution within the range of our study. The lattice constant $b$ slightly decreases with Ag/Pd doping, whereas, $a$ and $c$ increase with an overall increase in the unit cell volume. Both systems, CeCu${}_{6-x}$Ag${}_x$ and CeCu${}_{6-x}$Pd${}_x$, exhibit a magnetic quantum critical point (QCP), at $x$ $\approx$ 0.2 and $x$ $\approx$ 0.05 respectively. Near the QCP, long range antiferromagnetic ordering takes place at an incommensurate wave vector (${\delta }_1$ 0 ${\delta }_2$) where ${\delta }_1\sim 0.62$, ${\delta }_2\sim 0.25$, $x$ = 0.125 for CeCu${}_{6-x}$Pd${}_x$ and ${\delta }_1\sim 0.64$, ${\delta }_2\sim 0.3$, $x$ = 0.3 for CeCu${}_{6-x}$Ag${}_x$. The magnetic structure consists of an amplitude modulation of the Ce-moments which are aligned along the $c$-axis of the orthorhombic unit cell.

13 pages, 9 figures