A new form of NaMnAsO4

The new form of NaMnAsO4 (denoted as β) crystallizes in the monoclinic crystal system and is isotypic with one form of NaCoPO4 and with NaCuAsO4.

A new form of NaMnAsO 4 , sodium manganese(II) orthoarsenate, has been obtained under hydrothermal conditions, and is referred to as the -polymorph. In contrast to the previously reported orthorhombic -polymorph that crystallizes in the olivine-type of structure and has one manganese(II) cation in a distorted octahedral coordination, the current -polymorph contains two manganese(II) cations in [5]-coordination, intermediate between a squarepyramid and a trigonal bipyramid. In the crystal structure of -NaMnAsO 4 , four [MnO 5 ] polyhedra are linked through vertex-and edge-sharing into finite {Mn 4 O 16 } units strung into rows parallel to [100]. These units are linked through two distinct orthoarsenate groups into a framework structure with channels propagating parallel to the manganese oxide rows. Both unique sodium cations are situated inside the channels and exhibit coordination numbers of six and seven. -NaMnAsO 4 is isotypic with one form of NaCoPO 4 and with NaCuAsO 4 .

Chemical context
Magnussonite is a rare manganese(II) arsenite mineral and has been described with an ideal formula of Mn II 10 As III 6 O 18 (OH,Cl) 2 (Moore & Araki, 1979). In a recent project on hydrothermal crystal growth of phases in the system Mn II /As III /O (Priestner et al., 2018a) and a precise structure refinement of magnussonite, it could be shown that the obtained synthetic material has a composition of Mn II 3 As III 2 O 6 Á1/3H 2 O whereas naturally occurring material (type locality Lå ngban, Sweden) is better described as Mn II 3 As III 2 O 6 (Cu II (OH,Cl) 2 ) x (Priestner et al., 2018b). Building on that knowledge, a subsequent project was started to incorporate divalent transition-metal cations under hydrothermal conditions into synthetic magnussonite for obtaining similar compositions to those in the natural material. In one of the batches, containing manganese(II) acetate, sodium hydroxide, nickel chloride and arsenic(III) oxide as the arsenic source, we observed a partial oxidation of arsenic to yield monoclinic NaMnAsO 4 as a by-product with arsenic in an oxidation state of +V. NaMnAsO 4 was reported previously, as obtained from a high-temperature synthesis in a molten salt medium (Ulutagay-Kartin et al., 2002). This form crystallizes in the orthorhombic system with space-group type Pnma and adopts an olivine-type of structure.
In the following, we refer to the previously reported orthorhombic polymorph (Ulutagay-Kartin et al., 2002) as the -form, and the new monoclinic polymorph as the -form of NaMnAsO 4 . ISSN 2056-9890 2. Structural commentary -NaMnAsO 4 crystallizes isotypically with one of the three modifications of the phosphate NaCoPO 4 (Feng et al., 1997) and with the copper analogue NaCuAsO 4 (Ulutagay- Kartin et al., 2003). The asymmetric unit of -NaMnAsO 4 comprises of two formula units, and the principal building units are two manganese(II) cations in [5]-coordination, two orthoarsenate anions AsO 4 3-, and two sodium cations in a six-and sevenfold coordination by oxygen (Fig. 1).
The previously reported -form of NaMnAsO 4 has a calculated X-ray density D x = 4.03 g cm À3 and thus is denser than the current -form (3.95 g cm À3 ). Based on the rule of thumb that the denser polymorph is (in the majority of cases) the stable form, these values point to -NaMnAsO 4 as the thermodynamically stable polymorph. This assumption is supported by the preparation conditions of the different polymorphs. The -polymorph was obtained under hightemperature conditions (Ulutagay-Kartin et al., 2002) whereas the -polymorph crystallized under much milder temperature conditions. As a result of the scarcity of -NaMnAsO 4 material, a detailed investigation of the thermal behaviour was not conducted. However, a possible ! phase transition would be of the reconstructive type because the building units in the two structures exhibit a completely different arrangement.

Figure 1
The crystal structure of -NaMnAsO 4 in a projection along structures. This is supported by the similar 5 values of 0.57 and 0.47 for the two copper(II) cations in NaCuAsO 4 .

Synthesis and crystallization
A stoichiometric mixture of Mn(CH 3 COO) 2 Á4H 2 O, NaOH, As 2 O 3 and NiCl 2 in the ratio 1:6:1:1/6 was loaded in a Teflon container that was filled with 3 ml of water to two-thirds of its volume. Then the container was sealed with a Teflon lid and placed in a steel autoclave that was heated at 483 K for five days. After cooling to room temperature, the solid material was filtered off, washed with mother liquor, water and ethanol and air-dried. The main phase identified by single crystal and powder X-ray diffraction was synthetic magnussonite, Mn 3 As 2 O 6 Á1/3H 2 O (Priestner et al., 2018b). Synthetic sarkinite, a basic manganese(II) arsenate(V) with formula Mn 2 AsO 4 (OH) (Stock et al., 2002), and the title compound were also present as minor by-products, with -NaMnAsO 4 typically appearing in the form of needles.

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 2. Coordinates of isotypic NaCuAsO 4 (Ulutagay-Kartin et al., 2003) were standardized using the program STRUCTURE-TIDY (Gelato & Parthé, 1987) and then used as starting parameters for refinement. Free refinement of the site occupation factors for the two Mn sites resulted in a value of 1.000 (3) in each case, thus revealing no incorporation of Ni at these sites. Computer programs: APEX3 and SAINT (Bruker, 2015), SHELXL2017 (Sheldrick, 2015), ATOMS (Dowty, 2006) and publCIF (Westrip, 2010