# Terahertz spectroscopy for analysis of vaterite-to-calcite crystal phase transition induced by distilled water

## TL;DR

This paper shows how terahertz spectroscopy can track the transformation of vaterite to calcite in a non-destructive way.

## Contribution

The study introduces terahertz spectroscopy as a novel non-invasive method to quantify vaterite-to-calcite phase transitions.

## Key findings

- Terahertz absorption near 3.3 THz correlates linearly with vaterite concentration (R² = 0.934).
- The method enables non-destructive evaluation of crystal phase transitions.
- Terahertz spectroscopy is suitable for monitoring vaterite stability and transformation.

## Abstract

Vaterite is a crystalline polymorph of anhydrous calcium carbonate (CaCO3), which exhibits relatively low stability compared to other two polymorphs, calcite and aragonite. Vaterite particles have properties such as large specific surface area, high porosity, and high solubility; hence, research has been made in wide range of applications from material additive to drug delivery. X-ray diffractometry (XRD) is capable of identifying polymorphs of calcium carbonate. However, depending on the base material and the installed state, it may not always be suitable for non-destructive observation. This study reveals that vaterite-to-calcite crystal phase transition degree can be quantified by the absorption spectrum in terahertz range. The vaterite concentration and the terahertz absorption peak intensity near 3.3 THz showed linearity with correlation coefficient of R2 = 0.934 in our experiment. The findings allow us to quantitatively evaluate vaterite-to-calcite crystal phase transition in non-destructive and non-invasive way.