**1. Introduction**

The MgO-TiO2 ceramic system includes three stoichiometric magnesium titanate phases: Mg2TiO4 (qandilite), MgTiO3 (geikielite) and MgTi2O5 (karrooite) [1]. Their crystal structures have been determined by X-ray diffraction and neutron diffraction [2,3]. The phases are:


High-temperature X-ray diffraction (HT-XRD) studies have been published for all three stoichiometric Mg titanates prepared by the sol-gel technique during their transformation from the xerogel precursor to their final oxide form for a range of temperatures between 700 and 1300 ◦C [4]. High-temperature neutron diffraction (HT-ND) studies have been made on Mg2TiO4 synthesized by the solid-state reaction at temperatures between 90 to 1400 ◦C [5]. In this study, there was agreemen<sup>t</sup> between the lattice parameter of the HT-XRD and HT-ND data in the temperature range between 800 and 1300 ◦C [6]. Importantly, the in-situ HT-XRD studies of MgTiO3 during the firing of xerogel powders between 700 and 1300 ◦C yielded linear thermal expansion coefficients (TECs).

There are two additional publications with ND-XRD data for MgTiO3, which by contrast, showed non-linear TECs [7,8]. Both publications are for the same sample -Kar2-, studied where the MgTi2O5 (karrooite) phase was the major component and the MgTiO3 (geikielite) and TiO2 (rutile) phases were the minor components (by mass). In these publications, the phases were obtained by the solid-state reaction between MgCO3 and TiO2 fine powders, and the HT-ND data [7,8] were collected on sample Kar2 between 23 and 1305 ◦C.

Further examination of the HT diffraction works on geikielite showed that there is a significant gap in the lattice parameters between the HT-XRD data received from sol-gel product [4], and HT-ND data received from geikielite using the solid state reaction in that the lattice parameters of the sol-gel product were slightly lower. A key difference in the collection of the data was that the sol-gel data were obtained using a different sample at each temperature after 1 h firing, and the solid-state version was a single sample for all temperatures. In order to determine if the gap originated from the experimental methods or inherent differences in the synthesis (sol-gel vs. solid state), it was decided to investigate a single geikielite sample determining lattice parameters between RT up to 900 ◦C.

In this work, we complete the HT crystallographic data of sol-gel product of MgTiO3 using HT-XRD measurements of MgTiO3 made by sol-gel technique between 25 and 900 ◦C and provide insight to the differences in TEC behavior.

#### **2. Materials and Methods**
