Dedicated to Professor Rudolf Hoppe on the occasion of his 90th birthdayThe prediction and identification of stable and particularly of metastable compounds is important in achieving innovative materials. [1] With this objective in mind, approaches containing both theoretical examinations of phase stabilities and concepts of a rational synthesis [2] are gaining increasing importance. Thus, an investigation of energy landscapes [3] provides important information about local and global minima to predict the existence of new compounds and structures.Herein, we introduce a combination of quantum-chemical calculations and thermodynamic considerations to realize target-oriented planning and optimization of chemical synthesis. The analysis of phase formation is acquired with an in situ method for monitoring gas-phase reactions. Using the system P-As, we investigated a textbook example of monotropic phase transitions, which features a variety of known and postulated allotropes. [4] To estimate the relative stabilities of the compounds under discussion, structure allotropes of N, P, and As were modeled by means of DFT calculations. The calculated electronic energies (normalized to one atom Pn) for molecular Pn 2 and Pn 4 , the black/orthorhombic (o-Pn), gray/ trigonal (t-Pn), and the simple-cubic (c-Pn) allotrope as well as the tubular polymeric forms of Hittorf (H-Pn), [5] Ruck (R-Pn), [6] and Pfitzner (P-Pn), [7] which are known for phosphorus, are shown in Figure 1. The high stability of N 2 can be observed as well as the preference for solid-state structures for P and As. Figure 1 b emphasizes the results for the t and o forms of P and As. The known stability of o-P compared to the high-pressure modifications t-P and c-P is correctly predicted as well as the stability of gray t-As compared to the known high-pressure modification c-As, hypothetical tubular As allotropes, and predicted o-As. Does this mean that o-As can be synthesized as a metastable compound? [8a] The calculated values of the total electronic energies correctly express the higher stability of o-P compared with the high-pressure phase t-P and also t-As compared to o-As. From the computed values DE el = E el (o)ÀE el (t) of À4 kJ mol À1 for P and + 2.5 kJ mol À1 for As, a transition from the o-to the tstructure in the ideal solution As x P 1Àx can be estimated to be about x = 0.6 ( Figure 2). Taking thermodynamic energy terms into consideration (for details, see the Supporting Information), a distinctive stabilization of the o phase is calculated and the o-t transition shifts to a higher As content (x = 0.9). According to the calculated values, very low energy differences decide whether or not o-As can actually be synthesized. [8b]

The new phases Ca(3)Pt(4+x)Ge(13-y) (x = 0.1; y = 0.4; space group I2(1)3; a = 18.0578(1) Å; R(I) = 0.063; R(P) = 0.083) and Yb(3)Pt(4)Ge(13) (space group P4(2)cm; a = 12.7479(1) Å; c = 9.0009(1) Å; R(I) = 0.061, R(P) = 0.117) are obtained by high-pressure, high-temperature synthesis and crystallize in new distortion variants of the Pr(3)Rh(4)Sn(13) type. Yb(3)Pt(4)Ge(13) features Yb in a temperature-independent non-magnetic 4f(14) (Yb(2+)) configuration validated by X-ray absorption spectra and resonant inelastic X-ray scattering data. Ca(3)Pt(4+x)Ge(13-y) is diamagnetic (χ(0) = -5.05 × 10(-6) emu mol(-1)). The Sommerfeld coefficient γ = 4.4 mJ mol(-1) K(-2) for Ca(3)Pt(4+x)Ge(13-y), indicates metallic properties with a low density of states at the Fermi level in good agreement with electronic structure calculation (N(E(F)) = 3.3 eV(-1)/f.u.)); the Debye temperature (θ(D)) is 398 K.

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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