The crystal structure of TlMgCl3 from 290 K to 725 K

The title compound, thallium magnesium trichloride, has been identified as a scintillator with both moderate gamma-stopping power and moderate light yield. Knowledge of its crystal structure is needed for further development. This work determines the crystal structure of TlMgCl3 to be hexagonal P63/mmc (No. 194) and isostructural with RbMgCl3, contrary to previously reported data. Extending neutron diffraction measurements to high temperature, the data show that TlMgCl3 maintains this crystal structure from 290 K up through 725 K, approaching the melting point of 770 K.


Chemical context
In the ongoing search for inorganic scintillators with high gamma-stopping power, TlMgCl 3 has been identified. As a result of the presence of thallium, TlMgCl 3 has a high effective atomic number, Z eff = 67 [calculation methodology (Derenzo & Choong, 2009) in the supporting information], and a moderate density, = 4.47 g cm À3 (determined in this work). A pair of initial crystal growths of TlMgCl 3 have been conducted to assess the scintillation properties: Fujimoto et al. (2016) measured 46,000 ph MeV À1 light yield with 5% energy resolution at 662 keV, and Hawrami et al. (2017) measured 30,600 ph MeV À1 light yield with 3.7% energy resolution at 662 keV.
To develop this compound further, a precise determination of the crystal structure is necessary. This will enable firstprinciples calculations of the electronic configuration and may be useful in assessing challenges that arise during synthesis (e.g. from thermal stresses). This work reports the crystal structure of TlMgCl 3 between 290 K and 725 K, approaching the melting point of 770 K. Previous work on TlMgCl 3 by Beznosikov (1978) used powder diffraction to report the space group at room temperature as orthorhombic (a = 6.54, b = 9.22, c = 6.99 Å ). However, despite using the same synthesis procedure, the structure reported by Beznosikov does not fit the diffraction data reported herein. Arai et al. (2020) published diffraction data but did not provide information on the crystal structure.

Structural commentary
Single crystal X-ray diffraction (SC-XRD) determined TlMgCl 3 to have a hexagonal structure (space group P6 3 /mmc, No. 194) with lattice parameters a = 7.0228 (4), c = 17.4934 (15) Å at 290 K. Fig. 1 visualizes the unit cell, which shows a three-dimensional corner-and face-sharing framework of six-coordinated Mg atoms encapsulating the 12coordinated Tl atoms. There are six formula units in the unit cell. There are two thallium, two magnesium and two chlorine atoms in the asymmetric unit of TlMgCl 3 , with site symmetries of 6m2 and 3m; 3m and 3m; mm2 and m, respectively; key bond distances and angles are listed in Table 1. Pairs of Mg2centered octahedra share faces (via 3 Â Cl1) and these octahedral pairs share corners (via Cl2) with the Mg1 octahedra to generate an ABACBC hexagonal stacking sequence of the chloride ions in the c-axis direction with the thallium cations occupying the vacant 12-coordinate sites. The coordination polyhedra of the chloride ions are distorted ClMg 2 Tl 4 octahedra with the Mg 2+ ions in a cis disposition for Cl1 and a trans disposition for Cl2. The title compound is isostructural with RbMgCl 3 as reported by Devaney et al. (1981) and RbMnCl 3 as reported by Goodyear et al. (1977), who describe the structure in more detail. This structure is more complex than that of CsMgCl 3 (McPherson et al., 1970), which also has space group P6 3 /mmc but only requires two formula units per unit cell and has an AB hexagonal stacking sequence of the chloride ions in the c-axis direction.
Neutron diffraction (ND) conducted on powder samples produced diffraction patterns that were in agreement with the crystal structure determined by SC-XRD. Neutron diffraction was conducted at temperatures ranging from 300 K to 725 K.
TlMgCl 3 maintains the same P6 3 /mmc crystal structure over this measured temperature range (see supporting information for more details on the powder ND data and fits).  Table 1 Selected geometric parameters (Å , ).

Figure 2
The hexagonal lattice parameters of TlMgCl 3 as a function of temperature, from neutron diffraction data. Vertical error bars from Rietveld fitting are within the size of the symbols and are omitted. The dashed lines are second-order polynomial fits to the data.

Figure 1
The unit cell of TlMgCl 3 , with the MgCl 6 octahedra shown in polyhedral representation.

Figure 3
Thermal expansion coefficients as a function of temperature, calculated from the second-order polynomial fit of the lattice parameters in Fig. 2. (Fig. 3). The thermal expansion is greater along the a axis than the c axis. Besides the anisotropy in the lattice parameters, the atomic positions did not vary significantly with temperature, and therefore the bond lengths change with temperature as dictated by the lattice parameters alone.

Synthesis and crystallization
Crystals of TlMgCl 3 were grown from the melt using the vertical Bridgman method. High purity beads of TlCl and MgCl 2 were combined in a stoichiometric ratio and sealed in a quartz ampoule under vacuum (10 À6 Torr). The crystal was grown with a translation speed of 0.5 mm h À1 and was cooled over 72 h. To protect the moisture-sensitive reactants and products, all preparations before and after synthesis were conducted inside an argon-filled glove box.

Refinement
SC-XRD was conducted on a Bruker Kappa APEXII CCD diffractometer. The crystal was protected from moisture by oil during mounting and by an Oxford dry nitrogen gas cryostream system during data collection at 290 K. Crystal data, data collection and structure refinement details are summarized in Table 2. Powder high-temperature ND measurements were obtained using the high-pressure preferred orientation (HIPPO) neutron diffractometer at the short-pulsed spallation neutron source of the Lujan Neutron Scattering Center at Los Alamos National Laboratory (Wenk et al., 2003; Vogel et al., 2004). Powder samples were sealed under argon in vanadium tubes to protect from moisture during data collection. Time-of-flight data were collected with HIPPO detector panels of 3 He detector tubes arranged on five rings with nominal diffraction angles of 2 = 39, 60, 90, 120, and 144 . Count times were 90 minutes per dwell time. ND data were analyzed for all five rings simultaneously using the Rietveld method implemented in the GSAS code (Larson & Von Dreele, 2004) and automated by scripts through gsaslanguage (Vogel, 2011). To yield reliable absolute lattice parameters, the DIFC instrument calibration parameters were fitted for the room-temperature data using the lattice parameters from SC-XRD and were kept constant for the rest of the ND data at higher temperatures. For more details on the data collection and refinement of these neutron diffraction data, see Onken et al. (2018).