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Register a new userEffect of structure and composition on the electronic excitation induced amorphization of La$_2$Ti$_{2-x}$Zr$_x$O$_7$ ceramics
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Understanding the response of ceramics operating in extreme environments is of interest for a variety of applications. Ab initio molecular dynamic simulations have been used to investigate the effect of structure and $B$-site (=Ti, Zr) cation composition of lanthanum-based oxides (La$_2$$B_2$O$_7$) on electronic-excitation-induced amorphization. We find that the amorphous transition in monoclinic layered perovskite La$_2$Ti$_2$O$_7$ occurs for a lower degree of electronic excitation than for cubic pyrochlore La$_2$Zr$_2$O$_7$. While in each case the formation of O$_2$-like molecules drives the structure to an amorphous state, an analysis of the polyhedral connection network reveals that the rotation of TiO$_6$ octahedra in the monoclinic phase can promote such molecule formation, while such octahedral rotation is not possible in the cubic phase. However, once the symmetry of the cubic structure is broken by substituting Ti for Zr, it becomes less resistant to amorphization. A compound made of 50% Ti and 50% Zr (La$_2$TiZrO$_7$) is found to be more resistant in the monoclinic than in the cubic phase, which may be related to the lower bandgap of the cubic phase. These results illustrate the complex interplay of structure and composition that give rise to the radiation resistance of these important functional materials.
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The Raman spectroscopy and AC and DC magnetization of Dy$_{2-x}$Eu$_x$Ti$_2$O$_7$ have been investigated. In Raman Spectroscopy, the systematic shift in all phonon modes with Eu content in Dy$_{2-x}$Eu$_x$Ti$_2$O$_7$ confirms that Dy$^{3+}$ ion is substituted by Eu3+ ions. High concentration of Eu induces the dipolar exchange interactions and crystal-field interactions in Dy$_{2-x}$Eu$_x$Ti$_2$O$_7$. Rich Eu content samples (x=1.8 and 1.9) show the existence of wasp-waisted hysteresis loop and that can be attributed to the coexistence of dipolar field and anisotropy exchange interaction. AC susceptibility shows two single ion spin freezing transitions corresponding to Dy$^{3+}$ and Eu$^{3+}$ ions respectively in x = 1.5, 1.8, 1.9 samples.
By combining neutron scattering and magnetization measurements down to 80 mK, we determine the $(H,T)$ phase diagram of the Nd$_2$(Zr$_{1-x}$Ti$_x$)$_2$O$_7$ pyrochlore magnet compounds. In those samples, Zr is partially substituted by Ti, hence tuning the exchange parameters and testing the robustness of the various phases. In all samples, the ground state remains all in / all out, while the field induces phase transitions towards new states characterized by 2 in - 2 out or 1 out - 3 in / 1 in - 3 out configurations. These transitions manifest as metamagnetic singularities in the magnetization vs field measurements. Strikingly, it is found that moderate substitution reinforces the stability of the all in / all out phase: the Neel temperature, the metamagnetic fields along with the ordered magnetic moment are higher in substituted samples with $x <$ 10%.
Dy$_{2}$Zr$_{2}$O$_{7}$ a disordered pyrochlore system, exhibits the spin ice freezing under the application of magnetic field. Our studies suggest the stabilization of pyrochlore phase in Dy$_{2-x}$La$_{x}$Zr$_{2}$O$_{7}$ with the substitution of nonmagnetic La, along with the biphasic mixture for the intermediate compositions. We observed that the higher La compositions (1.5 $leq$ x $leq$ 1.9), show spin freezing (T $sim$ 17 K) similar to the field induced spin ice freezing for low La compositions (0 $leq$ x $leq$ 0.5), and the well known spin ice systems Dy$_{2}$Ti$_{2}$O$_{7}$ and Ho$_{2}$Ti$_{2}$O$_{7}$. The low temperature magnetic state for higher La compositions (1.5 $leq$ x $leq$ 1.9) culminates into spin glass state below 6 K. The Cole-Cole plot and Casimir-du Pr$acute{e}$ fit shows narrow distribution of spin relaxation time in these compounds.
There are reasons to believe that the ground state of the magnetic rare earth pyrochlore Yb$_2$Ti$_2$O$_7$ is on the boundary between competing ground states. We have carried out $mathrm{textit{ab initio}}$ density functional calculations to determine the most stable chemical formula as a function of the oxygen chemical potential and the likely location of the oxygen atoms in the unit cell of the stuffed system. We find that it is energetically favorable in the stuffed crystal (with an Yb replacement on a Ti site) to contain oxygen vacancies which dope the Yb 4$mathrm{textit{f}}$ orbitals and qualitatively change the electronic properties of the system. In addition, with the inclusion of the contribution of spin-orbit-coupling (SOC) on top of the GGA+U approach, we investigated the electronic structure and the magnetic moments of the most stable stuffed system. In our determined stuffed structure the valence bands as compared to those of the pure system are pushed down and a change in hybridization between the O 2$mathrm{textit{p}}$ orbitals and the metal ion states is found. Our first-principle findings should form a foundation for effective models describing the low-temperature properties of this material whose true ground state remains controversial.
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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