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This work presents a comprehensive analysis of the structural and vibrational properties of the kesterite CuZnSnS (CZTS, I4̅ space group) as well as its polymorphs with the space groups P4̅2c and P4̅2m, from both experimental and theoretical point of views. Multiwavelength Raman scattering measurements performed on bulk CZTS polycrystalline samples were utilized to experimentally determine properties of the most intense Raman modes expected in these crystalline structures according to group theory analysis. The experimental results compare well with the vibrational frequencies that have been computed by first-principles calculations based on density functional theory. Vibrational patterns of the most intense fully symmetric modes corresponding to the P4̅2c structure were compared with the corresponding modes in the I4̅ CZTS structure. The results point to the need to look beyond the standard phases (kesterite and stannite) of CZTS while exploring and explaining the electronic and vibrational properties of these materials, as well as the possibility of using Raman spectroscopy as an effective technique for detecting the presence of different crystallographic modifications within the same material.

Alkali metal doping is essential to achieve highly efficient energy conversion in Cu(In,Ga)Se 2 (CIGSe) solar cells. Doping is normally achieved through solid state reactions, but recent observations of gasphase alkali transport in the kesterite sulfide (Cu 2 ZnSnS 4 ) system (re)open the way to a novel gas-phase doping strategy. However, the current understanding of gas-phase alkali transport is very limited. This work (i) shows that CIGSe device efficiency can be improved from 2% to 8% by gas-phase sodium incorporation alone, (ii) identifies the most likely routes for gas-phase alkali transport based on mass spectrometric studies, (iii) provides thermochemical computations to rationalize the observations and (iv) critically discusses the subject literature with the aim to better understand the chemical basis of the phenomenon. These results suggest that accidental alkali metal doping occurs all the time, that a controlled vapor pressure of alkali metal could be applied during growth to dope the semiconductor, and that it may have to be accounted for during the currently used solid state doping routes. It is concluded that alkali gas-phase transport occurs through a plurality of routes and cannot be attributed to one single source.Control of alkali doping is crucial for a range of technologically relevant chalcogenide materials, from photovoltaics (CdTe, Cu(In,Ga)Se 2 , Cu 2 ZnSn(S,Se) 4 ) 1-5 and thermoelectricity (Pb(S,Se,Te)) 6-9 potentially to superconductivity (KFeSe 2 ) 10,11 and quantum computing (Bi 2 Te 3 12 , MoS 2 and WSe 2 13 ). In the case of Cu(In,Ga)Se 2 (CIGSe) solar cell material, the current alkali metal doping procedures are overwhelmingly based on condensed state reactions. Two common approaches are taken. Either by indirect control of the diffusion from a sodium-containing substrate or back contact [14][15][16][17] , or by deliberate doping from the precursor surface through a post deposition treatment (PDT), e.g. by NaF or KF evaporation onto the surface of the absorber to form a tens of nanometer thick layer, followed by annealing [18][19][20] . Control of the sodium content in the former case is difficult as substrates are never identical 21 , and in the latter case at least one extra step is required to add the alkali metal. The subject has been extensively reviewed by Salomé et al. 22. CIGSe thin films are always grown in a controlled atmosphere containing a certain pressure of selenium. The semiconductor requires selenium for its formation and to prevent its decomposition, given that the reaction is ruled by a solid/gas-phase equilibrium 23,24 . The question arises, are any other gas-phase chemical species involved in the equilibrium? All the main binary compounds of CIGSe have low vapour pressures; however, usually CIGSe contains also a considerable amount of sodium incorporated in the film. Is the vapour pressure of sodium or its

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Titanium alloys exhibit excellent corrosion resistance in most aqueous media due to the formation of a stable oxide film, and some of these alloys (particularly Ti-6Al-7Nb) have been chosen for surgical and odontological implants for their resistance and biocompatibility. Treatment with fluorides (F(-)) is known to be the main method for preventing plaque formation and dental caries. Toothpastes, mouthwashes, and prophylactic gels can contain from 200 to 20,000 ppm F(-) and can affect the corrosion behaviour of titanium alloy devices present in the oral cavity. In this work, the electrochemical corrosion behaviour of Ti-1M alloys (M = Ag, Au, Pd, Pt) was assessed in artificial saliva of pH = 3.0 containing 910 ppm F(-) (0.05 M NaF) through open circuit potential, E(OC), and electrochemical impedance spectroscopy (EIS) measurements. The corrosion behaviour of the Ti-6Al-7Nb commercial alloy was also evaluated for comparison. E (OC) measurements show an active behaviour for all the titanium alloys in fluoridated acidified saliva due to the presence of significant concentrations of HF and HF(2) (-) species that dissolve the spontaneous air-formed oxide film giving rise to surface activation. However, an increase in stability of the passive oxide layer and consequently a decrease in surface activation is observed for the Ti-1M alloys. This behaviour is confirmed by EIS measurements. In fact, the Ti-6Al-7Nb alloy exhibits lower impedance values as compared with Ti-1M alloys, the highest values being measured for the Ti-1Au alloy. The experimental results show that the corrosion resistance of the studied Ti-1M alloys is similar to or better than that of Ti-6Al-7Nb alloy currently used as biomaterial, suggesting their potential for dental applications.