## 1 Introduction  
Oxides, hydrides and hydroxides are common material classes that have been known and used for ages. Oxhydrides, on the other hand, make up a less known class of multi-anion materials where the oxide and hydride anions are sharing the same sites within the crystal lattice [1]. The first report on oxyhydride synthesis (namely LaH\({}_{1+2x}\)O\({}_{1-x}\) and LaH\({}_{1+y}\)O\({}_{1-x}\) (y \(<\) 2x) oxyhydrides) was published 40 years ago [2] and dealt with relatively high skepticism. At that time, researchers assumed that under normal conditions, the coexistence of dissimilarly charged hard O\({}^{2-}\) anions and soft, polarizable, H\({}^{-}\) anions at the same sites of a crystalline solid was thermodynamically unlikely [3, 4]. Direct experimental confirmation of hydrogen entering into the oxide structure was and still remains a challenging task, because most of the traditionally used material analysis methods such as X-ray diffraction, energy dispersive spectroscopy, X-ray photoelectron spectroscopy and others can not observe hydrogen directly. Nevertheless, credible crystal structure study by combining neutron and X-ray diffraction techniques [5] unambiguously confirmed the unusual type of oxygen and hydrogen anion site sharing and the existence of oxyhydrides was eventually accepted by the whole scientific community. Many of these materials were obtained by partially oxidizing hydrides and, based on their crystal structure, could be described as oxygen-containing hydrides [6]. However, hydrogen-containing oxides were reported as well [4]. More recently, some authors suggested adopting the usage of the -HO ending for the oxyhydrides [7], thus discriminating them from the hydroxides (i.e., materials with -OH groups). Following the first reports of LaHO synthesis by [PERSON] et al. [2,5], some more oxyhydrides were synthesized and identified as well [8, 9, 10]. However, in general, the reports of oxyhydride studies remained scarce, and for years these materials had no practical applications. Approximately one decade ago, researchers started to realize that unique properties of hydrogen can lead to the advanced functionality of oxyhydride materials, and the situation started to change drastically [3]. The ionic radius of hydrogen anion (139.9 pm) is relatively similar to the ionic radius of oxygen anion (145.2 pm) [11]; however, at the same time, H\({}^{-}\) ion has two times smaller charge and is distinctly easier to polarize than the O\({}^{2-}\) ion [4]. Accordingly, it was assumed that if O\({}^{2-}\) were replaced by H\({}^{-}\) (or vice versa), this would not induce large disturbances to the crystal lattice, but could create uncommon electron energy level structures and could lead to interesting magnetic and optical properties [3, 4, 12]. This, in turn, potentially might have appealing applications in commercial products. Recently, it was reported that in some of the oxyhydrides, H\({}^{-}\) and O\({}^{2-}\) ions might have the possibility of interacting via valence charge states of the metal center [1]. In partially oxidized yttrium hydride and several other rare-earth (Gd, Dy, and Er) hydrides, reversible switching of optical transmittance under intense irradiation was observed [13]. This feature of yttrium oxyhydride is currently being tested for application in smart window devices [14]. Titanium is also recognized for its potential to form corresponding oxyhydride phases [15]. However, up to now, most of the efforts to alter the TiO\({}_{2}\) structure by hydrogen (both in powder and film form) have been focused on the reduction of TiO\({}_{2}\) and the formation of oxygen vacancies that are beneficial for photocatalytic applications. Meanwhile, direct synthesis of ternary titanium oxyhydrides from the constituting elements (namely, titanium oxygen and hydrogen) have not yet been reported, and the practical value of such oxyhydrides has not yet been determined.