The structural and the magnetic properties of ${\mathrm{CeCu}}_{6-x}{\mathrm{Ag}}_x$ $\left(0\le x\le 0.85\right)$ and ${\mathrm{CeCu}}_{6-x}{\mathrm{Pd}}_x$ $\left(0\le x\le 0.4\right)$ have been studied using neutron diffraction, resonant ultrasound spectroscopy (RUS), x-ray diffraction measurements, and first principles calculations. The structural and magnetic phase diagrams of ${\mathrm{CeCu}}_{6-x}{\mathrm{Ag}}_x$ and ${\mathrm{CeCu}}_{6-x}{\mathrm{Pd}}_x$ as a function of Ag/Pd composition are reported. The end member, ${\mathrm{CeCu}}_6$, undergoes a structural phase transition from an orthorhombic $\left(Pnma\right)$ to a monoclinic $\left(P{2}_1/c\right)$ phase at 240 K. In ${\mathrm{CeCu}}_{6-x}{\mathrm{Ag}}_x$, the structural phase transition temperature $\left(T_s\right)$ decreases linearly with Ag concentration and extrapolates to zero at $x_S$ $\approx 0.1$. The structural transition in ${\mathrm{CeCu}}_{6-x}{\mathrm{Pd}}_x$ 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, ${\mathrm{CeCu}}_{6-x}{\mathrm{Ag}}_x$ and ${\mathrm{CeCu}}_{6-x}{\mathrm{Pd}}_x$, 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 ${\mathrm{CeCu}}_{6-x}{\mathrm{Pd}}_x$ and ${\delta }_1\sim 0.64,{\delta }_2\sim 0.3,x=0.3$ for ${\mathrm{CeCu}}_{6-x}{\mathrm{Ag}}_x$. The magnetic structure consists of an amplitude modulation of the Ce moments which are aligned along the $c$ axis of the orthorhombic unit cell.

• Received 1 October 2015

DOI:https://doi.org/10.1103/PhysRevB.92.214421