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Synthesis, floating zone crystal growth and characterization of the Quantum Spin Ice $\rm Pr_2Zr_2O_7$ pyrochlore

Pyrochlore $\rm Pr^{3+}_{2+x}Zr^{4+}_{2-x}O_{7-x/2}$ samples in the form of both powders $(-0.02 \le x \le 0.02)$ and bulk single crystals have been studied to elucidate the dependence of their magnetic, compositional and structural properties on synthesis and growth conditions. All samples were characterized using X-ray diffraction, specific heat, and DC magnetization measurements. The crystals were also studied using the X-ray Laue technique and scanning electron microscopy. Increasing the Pr content for the $\rm Pr_{2+x}Zr_{2-x}O_{7-x/2}$ powders enlarged the lattice parameter, and resulted in systematic changes in magnetic susceptibility and specific heat. Stoichiometric and high quality single crystals of $\rm Pr_2Zr_2O_7$ were grown using the optical floating zone technique under a high purity static argon atmosphere, to avoid inclusions of Pr$^{4+}$ and limit Pr vaporization. Increasing the growth speed was found to significantly reduce Pr vaporization for better control of stoichiometry. Scanning electron microscopy provided direct evidence of spinodal decomposition during growth that is controllable via rotation rate. An intermediate rotation rate of 3-6 rpm was found to produce the best microstructure. The magnetic susceptibility of crystals grown at rates from 1-20 mm/hr revealed changes that were consistent with Pr vaporization. Further, we report indications of local off-centering of Pr$^{3+}$ ions from the ideal pyrochlore sites, similar to what is known for the trivalent cation in $\rm Bi_2Ti_2O_7$ and $\rm La_2Zr_2O_7$. The effect varies with Pr content and radically modulates the low temperature specific heat. Overall, the results clearly demonstrate important correlations between the growth conditions and physical properties of $\rm Pr_2Zr_2O_7$ crystals.