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Register a new userSynthesis, floating zone crystal growth and characterization of the Quantum Spin Ice $rm Pr_2Zr_2O_7$ pyrochlore
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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.
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We have investigated the effect of synthesis and growth conditions on the magnetic, structural, and compositional properties of pyrochlore oxide holmium titanate and demonstrate a method for growing high quality stoichiometric single crystals. A series of polycrystalline samples with various contents of Ti (-0.08 leqslant x leqslant 0.08, and nominal compositions of Ho2Ti2+xO7) were synthesized at different temperatures, and characterized using powder X-ray diffraction. The results show that synthesizing powders at a higher temperature of 1500 {deg}C yield single phase compounds. Ti deficient powders showed an increase of lattice constant due to stuffing (Ho into Ti positions), while Ti rich powders showed a decrease in lattice constant due to anti-stuffing (Ti into Ho positions). A post annealing in O2 was found to be necessary to accomplish the anti-stuffing process. Use of the conventional floating zone (FZ) technique introduced Ti deficiency, stuffing, and oxygen vacancies in the grown crystal. Growth of high structural quality and stoichiometric single crystals of Ho2Ti2O7 by the traveling solvent floating zone (TSFZ) is reported. AC susceptibility measurements revealed that the stoichiometric crystal shows a higher ice freezing temperature, indicating that crystal quality and stoichiometry play a key role on low temperature spin ice properties of this compound.
We report the optimized conditions for growing the high quality single crystals of candidate quantum spin-ice Pr2Hf2O7 using the optical floating-zone method. Large single crystals of Pr2Hf2O7 have been grown under different growth conditions using a four-mirror type optical floating-zone furnace and their microscopic structural differences have been probed by high-resolution synchrotron x-ray diffraction (SXRD). The SXRD data reveal that the crystals grown under fowing argon (~ 2 L/h) atmosphere with slightly off-stoichiometric (optimized) starting composition yields the highest quality crystals. The magnetic susceptibility, isothermal magnetization and heat capacity data of optimally grown crystals are presented.
Single crystals of PrNiO3 were grown under an oxygen pressure of 295 bar using a unique high-pressure optical-image floating zone furnace. The crystals, with volume in excess of 1 mm3, were characterized structurally using single crystal and powder X-ray diffraction. Resistivity, specific heat, and magnetic susceptibility were measured, all of which evidenced an abrupt, first order metal-insulator transition (MIT) at ~130 K, in agreement with previous literature reports on polycrystalline specimens. Temperature-dependent single crystal diffraction was performed to investigate changes through the MIT. Our study demonstrates the opportunity space for high fugacity, reactive environments for single crystal growth specifically of perovskite nickelates but more generally to correlated electron oxides.
Effects of the growth velocity on the crystal growth behavior of Bi_2Sr_2Ca_1Cu_2O_x (Bi-2212) have been studied by floating zone technique. The results show that a necessary condition for obtaining large single crystals along the c-axis is that the solid-liquid interface of a growing rod maintains a stable planar growth front. The planar liquid-solid growth interface tends to break down into a cellular interface, while the growth velocity is higher than 0.25 mm/h. Single crystals of up to 50x7.2x7 mm3 along the a-, b- and caxes have been cut in a 7.2 mm diameter rod with optimum growth conditions. Tconset is 91 K measured by magnetic properties measurement system (MPMS) for as-grown crystals. Optical polarization microscope and neutron diffraction show that the quality of the single crystals is good.
Terbium titanate (Tb$_2$Ti$_2$O$_7$) is a spin-ice material with remarkable magneto-optical properties. It has a high Verdet constant and is a promising substrate crystal for the epitaxy of quantum materials with the pyrochlore structure. Large single crystals with adequate quality of Tb$_2$Ti$_2$O$_7$ or any pyrochlore are not available so far. Here we report the growth of high-quality bulk crystals using the Czochralski method to pull crystals from the melt. Prior work using the automated Czochralski method has suffered from growth instabilities like diameter fluctuation, foot formation and subsequent spiraling shortly after the seeding stage. In this study, the volumes of the crystals were strongly increased to several cubic centimeters by means of manual growth control, leading to crystal diameters up to 40 mm and crystal lengths up to 10 mm. Rocking curve measurements revealed full width at half maximum values between 28 and 40 for 222 reflections. The specific heat capacity c$_p$ was measured between room temperature and 1573 K by dynamic differential scanning calorimetry and shows the typical slow parabolic rise. In contrast, the thermal conductivity kappa(T) shows a minimum near 700 K and increases at higher temperature T. Optical spectroscopy was performed at room temperature from the ultraviolet to the near infrared region, and additionally in the near infrared region up to 1623 K. The optical transmission properties and the crystal color are interpreted to be influenced by partial oxidation of Tb$^{3+}$ to Tb$^{4+}$.
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