Melting Behavior of Alkaline-Earth Metal Carbodiimides and Their

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Cite This: Inorg. Chem. 2019, 58, 8938−8942

Melting Behavior of Alkaline-Earth Metal Carbodiimides and Their Thermochemistry from First-Principles Akira Hosono,*,† Ralf Peter Stoffel,‡ Yuji Masubuchi,*,§ Richard Dronskowski,‡,∥ and Shinichi Kikkawa§ †

Graduate School of Chemical Science and Engineering, Hokkaido University, N13W8, Kita-ku, Sapporo 060-8628, Japan Chair of Solid-State and Quantum Chemistry, Institute of Inorganic Chemistry, RWTH Aachen University, 52056 Aachen, Germany § Faculty of Engineering, Hokkaido University, N13W8, Kita-ku, Sapporo 060-8628, Japan ∥ Hoffmann Institute of Advanced Materials, Shenzhen Polytechnic, 7098 Liuxian Boulevard, Nanshan District, Shenzhen 518055, China

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‡

S Supporting Information *

structures.5,6,22,23 In these, the alkaline-earth metal cations are coordinated by the nitrogen atoms of the carbodiimide units. A thermal transformation from rhombohedral β-SrNCN to orthorhombic α-SrNCN at ∼920 K was reported in both experimental and computational approaches.22 Rhombohedral BaNCN (r-BaNCN) was synthesized by the reaction of barium nitride with melamine at 1013 K.5 Tetragonal BaNCN (tBaNCN) was recently obtained by the ammonolysis of BaCO3 at 1173 K, and its crystal structure was studied.23,24 Each Ba2+ ion in r-BaNCN is coordinated by six nitrogen atoms, forming significantly distorted BaN6 octahedra. For t-BaNCN, the coordination number of Ba2+ is eight, resulting in a symmetric BaN8 square-antiprism. Interestingly enough, the thermodynamical stability of the BaNCN polymorphs has not been investigated. This is not surprising because their precise thermogravimetry (TG) is difficult because most of this kind of compounds decompose at high temperature. Also, their high reactivity against oxygen and humidity makes any such measurement quite difficult. On the contrary, density functional theory (DFT) calculations are useful to get a first clue in regards to the thermal properties of unstable and reactive materials.22 In this Communication, experimental and theoretical thermochemical studies were performed on a series of alkaline-earth metal carbodiimides. Their melting behaviors were discussed in relation to their thermal decomposition around their melting points. The stabilities of r- and t-BaNCN were also compared based on their theoretical Gibbs free energies, as derived from the quasi-harmonic phonon approximation. The white carbodiimide powders were prepared by the ammonolysis of their respective carbonates.20,21,23,24 The respective melting points of α-SrNCN and t-BaNCN were determined to be 1293 and 1183 K by differential thermal analysis (DTA) peaks, as already reported in our previous manuscript.24 The melt of α-SrNCN is unstable, and it rapidly decomposes to SrC2, metallic Sr, and elemental nitrogen. The melt of BaNCN, however, is stable, and its white solid is

ABSTRACT: We present a combined experimental and theoretical investigation targeted at the thermochemical properties of a series of alkaline-earth metal carbodiimides. Their Gibbs energies and decomposition temperatures were calculated on the basis of phonons derived from density functional theory. The theoretical decomposition temperatures arrive at 1270, 1224, and 1185 K for CaNCN, α-SrNCN, and tetragonal BaNCN, respectively. Only the melt of tetragonal BaNCN is maintained at ∼1173 K, which is slightly below its calculated decomposition temperature. Experimentally, the melt of BaNCN did not decompose below 1273 K. On the contrary, both CaNCN and α-SrNCN partially decompose by forming a mixture of their carbides, metals, and nitrogen. The calculated Gibbs energies also show that the tetragonal phase of BaNCN is more stable than the rhombohedral one. We conclude that the melt of BaNCN is useful in the crystal growth of oxynitride perovskites such as BaTaO2N.

S

olid-state research on metal carbodiimides and cyanamides has a long history, and their syntheses have been reported to date, including alkaline (Li, Na, K, Cs),1−4 alkaline-earth (Ca, Sr, Ba),5,6 main-group element (Mg, Si, Tl, Pb),5,7−9 transition metals (Cr, Mn, Fe, Co, Ni, Cu, Zn, Y, Ag, Cd),8,10−16 and rareearth metals (Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb).15,17 Recently, ternary metal carbodiimides such as SrZn(NCN)2 and LiM(NCN)2 (M = Al, In, Yb) have also been synthesized.18,19 These metal-containing carbodiimides are often composed of anions of symmetric [NCN]2− shape, to be distinguished from the asymmetric cyanamide anion [N−CN]2−. Many of them, especially the thermally unstable metal carbodiimides, have been synthesized by metathesis reactions between metal halides and carbodiimides.10−13,18 Also, ammonolysis was applied on metal carbonates.20,21 The latter route can be applied in the case when the starting metal carbonates are stable up to the nitridation temperature by ammonia (∼773 K). The alkaline-earth metal carbodiimides CaNCN, SrNCN, and BaNCN have been investigated in terms of their crystal © 2019 American Chemical Society

Received: May 21, 2019 Published: June 28, 2019 8938

DOI: 10.1021/acs.inorgchem.9b01462 Inorg. Chem. 2019, 58, 8938−8942

Communication

Inorganic Chemistry obtained after cooling to room temperature.24 The herepresented powderous white CaNCN was synthesized by heating ∼150 mg of CaCO3 powder (Wako Pure Chemical, 99.95%) in flowing ammonia to 873 K for 10 h. It contained 18 wt % of CaO as a secondary phase, as shown in Figure 1a. The crystalline

was oxidized to CaO by the oxygen from the air in picking up the product from the tube furnace. A trace amount of CaC2 was also detected in the black solidified product. There was no apparent change such as melting and solidification in the product heated to 1603 K. CaNCN was assumed to melt at 1613 K, as previously reported,28 and it decomposed to a mixture of CaC2, Ca metal, and nitrogen. The thermal decomposition was not completed at 1623 K, and a large amount of CaNCN remained. The side reaction was investigated by some researchers.27 The formation of a trace amount of Ca3N2 (1523 K,27 as in the present research at 1623 K, so the amount of Ca3N2 formed during heating was assumed to be small enough to ignore. A similar decomposition to CaNCN was observed in the case of α-SrNCN. As for BaNCN, a decomposition to a mixture of BaC2 and Ba metal was not observed below 1273 K, which is far above the experimentally determined melting point of 1183 K.24 Its decomposition temperature could not be experimentally determined because some of the BaNCN had been evaporated above its melting point.24 The thermal decomposition reaction of alkaline-earth metal carbodiimides can be expressed as follows

Figure 1. XRD patterns and photographs of the mixture of CaNCN and CaO. (a) As-prepared powder obtained by the ammonolysis of CaCO3 powder at 873 K for 10 h. (b) Ground powder of the black solid after heating to 1623 K for 5 h. Diamonds, arrows, and circles indicate CaNCN (ICSD 25763), CaO (PDF 00-037-1497), and CaC2 (ICSD 252755), respectively.

2Ae NCN → Ae + AeC2 + 2N2

(1)

where Ae indicates alkaline-earth metal. The decomposition temperatures of the metal carbodiimides can be calculated from the theoretical Gibbs energy values of reaction 1, as described below. DFT calculations were carried out by employing the program VASP (Vienna ab initio simulation package)29 to obtain optimized structures and electronic total energies. Plane-wave basis sets with an energetic cutoff of 500 eV were used. Exchange and correlation were treated following the generalized gradient approximation.30 To account for van-der-Waals-like interactions, the D3 method of Grimme was applied.31 All of the unit cells were allowed to change their volumes and shapes during the relaxation process but kept the initial symmetry. Phonon computations were conducted using the program PHONOPY32 based on the Hellmann−Feynman forces calculated using VASP. The supercells for the force calculations were set up by multiplying the unit cells in each direction such that the lattice parameters of the supercells were well above 10 Å. Thermodynamic potentials were obtained in the framework of the quasi-harmonic approximation.33,34 The calculations were conducted for the crystal structures of all of the alkaline-earth metal carbodiimides, alkaline-earth metals, graphite, and crystalline nitrogen.5,22,23,35−42 In the case of BaNCN, the Gibbs energies were calculated for both the rhombohedral and tetragonal phases before comparing the values among alkaline-earth metal carbodiimides. t-BaNCN was more stable than the r phase by ∼20 kJ/mol over the entire temperature range of 0 to 1500 K, as shown in Figure 2. In the previous report, there was no DTA signal, indicating the formation of r-BaNCN during heating. Nearly a single phase of r-BaNCN was obtained after cooling the melt of t-BaNCN at a rate of 10 K/min.24 However, t-BaNCN was contained in the products obtained in slow cooling of molten BaNCN at a rate of 2 K/h in our preliminary experiments, so the tetragonal phase seems to be the thermodynamically stable one. A similar phase transition of the nitrogen-containing compound influenced by the cooling rate is reported in the α and β phases of Li3BN2.43

phases were identified using powder X-ray diffraction (XRD, Ultima IV, Rigaku) with Cu Kα radiation over the 2θ range of 10 to 80° with a step size of 0.02° and a scanning rate of 10°/min. The amounts of CaNCN and CaO were estimated using Rietveld refinement.25 The preparation of phase-pure CaNCN was challenging because both the ammonolysis reaction and the decarboxylation of CaCO3 start at a similar temperature range (approximately 82320 and 883 K26). In our preliminary work, the nitridation temperature of 823 K was too low to obtain CaNCN-rich product, and a higher temperature of 873 K was necessary. Caution! Toxic gases such as cyan and carbon monoxide can be released during the sample preparation and its decomposition. The following processes should be conducted in a f ume food. TG measurements (STA2500, Netzsch) were conducted on powderous CaNCN in an alumina crucible at a heating rate of 10 K/min under a nitrogen flow (>99.999%). There was no significant weight change between 800 and 1273 K, as shown in Figure S1, although a slight weight loss related to a desorption of the surface adsorbate was observed below 800 K. TG at higher temperature was not conducted to avoid the possible damage to TG equipment by the previously reported decomposition of CaNCN.27 Additional thermal studies at temperature >1273 K were conducted by heating ∼100 mg of compacted powderous CaNCN in an alumina tube furnace. The CaNCN compacts with a diameter of 6 mm were fabricated by uniaxial pressing at 46 MPa; then, they were put on an alumina boat in a flowing nitrogen. The samples were heated to either 1603 or 1623 K for 5 h, which was slightly lower or higher than the reported melting point of CaNCN (1613 K),28 respectively. The sample changed from a white powder to a black solidified mass upon heating to 1623 K. It was still a mixture of CaNCN and CaO, but the CaO amount increased from the starting material, as depicted in Figure 1b. The Ca metal in the thermally decomposed product 8939

DOI: 10.1021/acs.inorgchem.9b01462 Inorg. Chem. 2019, 58, 8938−8942

Communication

Inorganic Chemistry

Figure 3. Gibbs energies of the decomposition reactions of alkalineearth metal carbodiimides as a function of the temperature. Red, green, and blue lines indicate CaNCN, α-SrNCN, and t-BaNCN, respectively.

Figure 2. Gibbs energy values of r- and t-BaNCN as a function of the temperature. Broken and solid lines are r and t phases, respectively.

More details of the transformation mechanism of BaNCN are now under investigation. Because the r phase with distorted BaN6 octahedra5 is less stable than the t phase,23 only the more stable t-BaNCN was taken into account in the following discussion. Pseudobonding dissociation energy values (Hbond) were calculated at 0 K and 0 Pa based on the total electronic energies E0 of AeNCN and single atoms of alkaline earth metal, carbon, and nitrogen

solid strongly depends on the lattice enthalpy values,46 and the decomposition temperatures change with the ionic size of alkaline-earth metals, similar to the melting points. The melts of CaNCN and α-SrNCN are unstable and decompose rapidly in the present experimental study. On the contrary, the calculated decomposition temperature of BaNCN is 1185 K, which is slightly higher than the experimental melting point (1183 K). As a consequence, only the melt of BaNCN is stable and can be used as a flux for crystal growth or as a sintering additive in alkaline-earth oxynitrides. In fact, single-crystalline Sr1−xBaxTaO2N (x = 0.04 to 0.23) with perovskite-type solid solution has been obtained by the reaction of molten BaNCN and SrTaO2N.24 In conclusion, t-BaNCN is more stable than r-BaNCN, and it melts at 1183 K, which is slightly below its theoretically estimated decomposition point of 1185 K. It is a promising flux for the crystal growth and sintering of nitrides and oxynitrides. Melts of other alkaline-earth carbodiimides are unstable and decompose into mixtures of their metals, carbides, and nitrogen. The tendency of their melting and decomposition points matched the results of the theoretical study.

Hbond = E0(Ae NCN) − {E0(Ae) + E0(C) + 2E0(N)} (2)

The calculated Hbond values are summarized in Table 1. These energies gradually decrease with the melting points in the order Table 1. Pseudo-Bonding Energy Values (Hbond) and Melting Points of Metal Carbodiimides compound

Hbond/kJ mol−1

melting point/K

CaNCN α-SrNCN t-BaNCN

2045 2014 2012

161328 129324 118324

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CaNCN, α-SrNCN, and t-BaNCN, together with increasing ionic radii of the alkaline-earth metal ions,24,28 as expected from a simple lattice energy picture. Going beyond pure electronic energies at 0 K and reaching finite T, the Gibbs energies of the crystalline compounds at high temperatures were obtained by the quasi-harmonic approximation in the standard procedure.33,34 The Gibbs free energy of gaseous nitrogen was constructed by summing up the electronic total energy E0, the vibrational zero-point Helmholtz free energy Aph,0 calculated for crystalline α-nitrogen,39 literature values of the sublimation enthalpy ΔHsub, the entropy S, and the heat capacity Cp(T) of gaseous nitrogen.28,44 The following equation was employed, as in a previous publication45 G(N2) = E0 + A ph,0 + ΔHsub − TS +

∫ Cp(T ) dT

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.9b01462. Figure S1. TG graph of CaNCN powder (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*A.H.: E-mail: [email protected]. *Y.M.: E-mail: [email protected]. ORCID

Akira Hosono: 0000-0002-8491-2941 Yuji Masubuchi: 0000-0003-3601-7077 Richard Dronskowski: 0000-0002-1925-9624 Shinichi Kikkawa: 0000-0002-3498-0735

(3)

The Gibbs energies for the decomposition reaction 1 are plotted in Figure 3. The point of ΔGr = 0 indicates the temperature where the spontaneous decomposition of the alkaline-earth metal carbodiimide occurs. Hence, the theoretical decomposition temperatures of CaNCN, α-SrNCN, and t-BaNCN are 1270, 1224, and 1185 K, respectively. CaNCN and α-SrNCN decompose below their experimental melting points, as shown in Table 1. It is well -known that the thermochemical behavior of a

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS A.H. was financially supported for his stay in Aachen by the International Collaborative Chemical Science Program (Scholarship for Short Term Visit) from the Graduate School of 8940

DOI: 10.1021/acs.inorgchem.9b01462 Inorg. Chem. 2019, 58, 8938−8942

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Chemical Sciences and Engineering, Hokkaido University, Japan. This research was partly supported by a KAKENHI Grant-in-Aid for Scientific Research(A) (grant no. 24245029), Research Fellow (grant no. 19J10301), and Scientific Research on Innovative Areas “Mixed Anion” (grant no. JP16H06439) from the Japan Society for the Promotion of Science (JSPS). We also thank the Jülich−Aachen Research Alliance (JARA) as well as the RWTH Aachen University IT Center for providing CPU time within the JARA-HPC project “jara0033”.

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DOI: 10.1021/acs.inorgchem.9b01462 Inorg. Chem. 2019, 58, 8938−8942