# Crystal growth and characterization of the pyrochlore Tb$_2$Ti$_2$O$_7$

## TL;DR

This study reports the successful growth of high-quality Tb$_2$Ti$_2$O$_7$$ crystals using a manual Czochralski method, enabling detailed characterization of their structural, thermal, and optical properties for potential quantum material applications.

## Contribution

It demonstrates a manual control approach to grow large, high-quality pyrochlore crystals overcoming previous growth instabilities.

## Key findings

- Crystals achieved up to 40 mm diameter and 10 mm length.
- Rocking curve FWHM between 28 and 40 arcseconds.
- Optical properties affected by partial oxidation of Tb ions.

## Abstract

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$^{4+}$.