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Although the formation of thallium hydrogen fluorides TlH x F x +1 ( x = 1, 1.5, 2, 3, 5, 6.5, and 7) from TlF and HF was reported in the 1970s, little is known about the corresponding crystal structures. To shed light on the crystal chemistry of the thallium fluorides, we reinvestigated the structure of α-TlHF 2 , which was obtained by crystallization of TlF from 50% HF at room temperature. We disclose that β-TlHF 2 is isostructural with α-MHF 2 (M = K, Rb, and Cs), γ-TlHF 2 is isostructural with β-MHF 2 (M = K and Rb), whereas α-TlHF 2 crystallizes in an unusual structure type. During the reaction of TlHF 2 with the glass of the microscope slide, crystals of the previously unknown compound Tl 2 [SiF 6 ]·2TlF were formed. We disclose that the related Tl 2 [GeF 6 ]·2TlF can be obtained by the crystallization of stoichiometric amounts of Tl 2 [GeF 6 ] with TlF from water at room temperature. The crystal structures of α-TlHF 2 , Tl 2 [SiF 6 ]·2TlF, Tl 2 [SiF 6 ]·TlCl, Tl 2 [GeF 6 ]·2TlF, Tl 2 [GeF 6 ]·TlCl, and Tl 2 [GeF 6 ] were determined by single-crystal X-ray diffraction analysis. The crystal structures of α-TlF and Tl 2 [SiF 6 ]· x TlF ( x = 0, 1) were redetermined at low temperatures.

The unusually high oxidation state + IV of cobalt is stabilized by ligands based on [B 11 H 11 ] 4- in dark blue colored Cs 4 [Co(B 11 H 10.11 (OH) 0.75 ) 2 ]·4.56H 2 O, K 4 [Co(B 11 H 9.19 (OH) 1.81 ) 2 ]·2H 2 O, Cs 8 [Co{(B 11 H 6 ) 2 (O)(O 2 BOH) 4 }] 2 ·4H 2 O and K 4 [Co{(B 11 H 6 ) 2 (O 2 BOH) 5 }]·7H 2 O. These compounds were obtained by reacting Co 2+ salts with [B 11 H 14 ] - under alkaline conditions. In the absence of oxygen, Co(+III) compounds such as the light brownish K 4 [Co(B 11 H 11 )(CN) 3 ]·KCl·2.5H 2 O are formed. The title compounds were characterized by X-ray crystallography. Cs 8 [Co{(B 11 H 6 ) 2 (O)(O 2 BOH) 4 }] 2 ·4H 2 O and K 4 [Co(B 11 H 11 )(CN) 3 ]·KCl·2.5H 2 O were also characterized using IR-, UV-vis and cyclovoltammetry. Magnetic measurements of Cs 4 [Co(B 11 H 10.11 (OH) 0.75 ) 2 ]·4.56H 2 O and ESR measurements of Cs 8 [Co{(B 11 H 6 ) 2 (O)(O 2 BOH) 4 }] 2 ·4H 2 O show that in these Co(+IV) low-spin d 5 complexes the unpaired electron is on the d x 2- y 2 , d xy ( E 2g ) orbitals.

Quaternary transition metal cyanamides Na 2 MSn 2 (NCN) 6 with M = Mn, Fe, Co, and Ni were prepared via solid-state metathesis reactions between Na 2 Sn(NCN) 3 and binary transition metal fluorides MF 2 in a 2:1 molar ratio. All phases crystallize isotypically in [NiAs]-derived structures ( P3̅ 1 m ) with inter- and intra-layer cation ordering over the octahedral sites. This leads to a highly asymmetric coordination of the NCN 2- anion, resulting in a strong degree of cyanamide character, which is confirmed via IR measurements. Intriguingly, the optical properties of Na 2 MSn 2 (NCN) 6 phases change markedly as the nature of the divalent transition metal is varied, and UV-vis measurements evidence a band gap reduction from Mn (3.43 eV) via Fe (1.90 eV) to Co (1.75 eV), which broadly mirrors the DFT+ U calculated energetic interval from the Fermi level to the unoccupied 3d states. Mott-Schottky analysis then goes on to characterize Na 2 FeSn 2 (NCN) 6 and Na 2 CoSn 2 (NCN) 6 as n -type semiconductors with flat-band potentials of 0.46 and -0.24 eV, respectively, vs RHE. This study demonstrates the utility of transition metal substitutions, within a flexible cyanamide framework, to electronically tune this growing family of pseudo-oxides.

Mixed-anion inorganic compounds offer diverse functionalities as a function of the different physicochemical characteristics of the secondary anion. The quaternary metal oxynitrides, which originate from substituting oxygen anions (O 2- ) in a parent oxide by nitrogen (N 3- ), are encouraging candidates for photoelectrochemical (PEC) water splitting because of their suitable and adjustable narrow band gap and relative negative conduction band (CB) edge. Given the known photochemical activity of LaTiO 2 N, we investigated the paramagnetic counterpart NdTiO 2+ x N 1- x . The electronic structure was explored both experimentally and theoretically at the density functional theory (DFT) level. A band gap ( E g ) of 2.17 eV was determined by means of ultraviolet-visible (UV-vis) spectroscopy, and a relative negative flat band potential of -0.33 V vs reversible hydrogen electrode (RHE) was proposed via Mott-Schottky measurements. 14 N solid state nuclear magnetic resonance (NMR) signals from NdTiO 2+ x N 1- x could not be detected, which indicates that NdTiO 2+ x N 1- x is berthollide, in contrast to other structurally related metal oxynitrides. Although the bare particle-based photoanode did not exhibit a noticeable photocurrent, Nb 2 O 5 and CoO x overlayers were deposited to extract holes and activate NdTiO 2+ x N 1- x . Multiple electrochemical methods were employed to understand the key features required for this metal oxynitride to fabricate photoanodes.

Bismuth(III) oxide-carbodiimide (Bi 2 O 2 NCN) has been recently discovered as a novel mixed-anion semiconductor, which is structurally related to bismuth oxides and oxysulfides. Given the structural versatility of these layered structures, we investigated the unexplored photochemical properties of the target compound for photoelectrochemical (PEC) water oxidation. Although Bi 2 O 2 NCN does not generate a noticeable photocurrent as a single photoabsorber, the fabrication of heterojunctions with the WO 3 thin film electrode shows an upsurge of current density from 0.9 to 1.1 mA cm -2 at 1.23 V vs reversible hydrogen electrode (RHE) under 1 sun (AM 1.5G) illumination in phosphate electrolyte (pH 7.0). Mechanistic analysis and structural analysis using powder X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), and scanning transmission electron microscopy energy-dispersive X-ray spectroscopy (STEM EDX) indicate that Bi 2 O 2 NCN transforms during operating conditions in situ to a core-shell structure Bi 2 O 2 NCN/BiPO 4 . When compared to WO 3 /BiPO 4 , the in situ electrolyte-activated WO 3 /Bi 2 O 2 NCN photoanode shows a higher photocurrent density due to superior charge separation across the oxide/oxide-carbodiimide interface layer. Changing the electrolyte from phosphate to sulfate results in a lower photocurrent and shows that the electrolyte determines the surface chemistry and mediates the PEC activity of the metal oxide-carbodiimide. A similar trend could be observed for CuWO 4 thin film photoanodes. These results show the potential of metal oxide-carbodiimides as relatively novel representatives of mixed-anion compounds and shed light on the importance of the control over the surface chemistry to enable the in situ activation.

Mössbauerite, a trivalent iron-only layered oxyhydroxide, has been recently identified as an electrocatalyst for water oxidation. We investigated the material as potential cocatalyst for photoelectrochemical water oxidation on semiconductor photoanodes. The band edge positions of mössbauerite were determined for the first time with a combination of Mott-Schottky analysis and UV-vis diffuse reflectance spectroscopy. The positive value of the Mott-Schottky slope and the flatband potential of 0.34 V vs reversible hydrogen electrode (RHE) identifies the material as an n-type semiconductor, but bare mössbauerite does not produce noticeable photocurrent during water oxidation. Type-II heterojunction formation by facile drop-casting with WO 3 thin films yielded photoanodes with amended charge carrier separation and photocurrents up to 1.22 mA cm -2 at 1.23 V vs RHE. Mössbauerite is capable of increasing the charge carrier separation at lower potential and improving the photocurrent during photoelectrochemical water oxidation. The rise in photocurrent of the mössbauerite-functionalized WO 3 photoanode thus originates from improved charge carrier separation and augmented hole collection efficiency. Our results highlight the potential of mössbauerite as a second-phase catalyst for semiconductor electrodes.