Structures and Phase Transitions in (MoO₂)₂P₂O₇

Abstract

We report structural investigations into (MoO(2))(2)P(2)O(7) using a combination of X-ray, neutron and electron diffraction, and solid-state NMR supported by first principles quantum chemical calculations. These reveal a series of phase transitions on cooling at temperatures of 377 and 325 K. The high temperature gamma-phase has connectivity consistent with that proposed by Kierkegaard at room temperature (but with improved bond length distribution), and contains 13 unique atoms in space group Pnma with lattice parameters a = 12.6577(1) A, b = 6.3095(1) A, c = 10.4161(1) A, and volume 831.87(1) A(3) from synchrotron data at 423 K. The low temperature alpha-structure was indexed from electron diffraction data and contains 60 unique atoms in space group P2(1)/c with cell parameters a = 17.8161(3) A, b = 10.3672(1) A, c = 17.8089(3) A, beta = 90.2009(2) degrees, and volume 3289.34(7) A(3) at 250 K. First principles calculations of (31)P chemical shift and J couplings were used to establish correlation between local structure and observed NMR parameters, and 1D and 2D (31)P solid-state NMR used to validate the proposed crystal structures. The intermediate beta-phase is believed to adopt an incommensurately modulated structure; (31)P NMR suggests a smooth structural evolution in this region.

Towards homonuclear J solid-state NMR correlation experiments for half-integer quadrupolar nuclei: experimental and simulated 11B MAS spin-echo dephasing and calculated 2JBB coupling constants for lithium diborate

“…Interestingly, calculations for lithium 65 diborate, lithium metaborate and lithium triborate reveal a clear trend whereby the calculated 2 J 11B-11B couplings increase from 0.95 to 3.23 Hz with increasing B-O-B bond angle (116º to 133º) and B-B distance (2.40 to 2.56 Å). A similar trend has been observed for the dependence on 2 J 29Si-29Si and 2 J 31P-70 31P couplings with respect to the Si-O-Si 98,108 and P-O-P 93 bond angle in silicates and phosphates; for silicates and phosphates, the bond angles are larger (>140º) and 2 J 29Si-29Si and 2 J 31P-31P couplings have been experimentally measured and utilised. 75 In conclusion, this paper has shown that, while 11 B spinecho dephasing times can be favourably long, it is the small 2 J 11B-11B couplings that are a consequence of the small B-O-B bond angles observed in borates that are hampering the development of 11 B homonuclear J correlation solid-state 80 NMR experiments.…”

“…9 in 115 Ref. 93 ) bond angle, respectively. Moreover, it is to be noted that the experimental observation of larger 2 J 29Si-29Si and 2 J 31P-31P couplings correspond to compounds exhibiting larger Si-O-Si (e.g., 3.6 to 8.0 Hz for angles between 139º and 150º in parawollastonite 98 and 6.3 to 23.5 Hz for angles between 137º 5 and 173º in the zeolite Sigma-2 108 ) and P-O-P (e.g., 9 to 30 Hz for angles between 139º to 157º in (MoO 2 ) 2 P 2 O 7 , see Tables 4 & 5 in Ref.…”

“…16 and Table 5 in Ref. 93 ). Longer spin-echo dephasing times of 55 24 to 100 ms have been determined for 17 O spin-echo MAS NMR of partially 17 O-labelled glycine.HCl and uracil, 20 with these longer dephasing times being of the same magnitude as those obtained from fits in this paper for lithium diborate at 5 and 25% 11 B abundance sample (see Table 3).…”

“…There is a growing literature of examples where homonuclear J couplings are determined from spin-echo MAS NMR experiments for spin I = 1/2 nuclei such as 13 C, 15 N, 29 Si and 31 P. 10,16,47,61,[93][94][95][96][97][98] In these cases, cosine modulation due to a J 65 coupling leads to clear zero crossings (at  = n/2J, where n = 1, 3,… for a 90  /2  180  /2  t acq pulse sequence) allowing the J coupling constants to be determined to a high accuracy. While there is still significant signal intensity for the B3 site at a spin-echo duration of 166.6 ms for the 11 B 70 MAS NMR spin-echo data in Fig.…”

“…Moreover, it is to be noted that the experimental observation of larger 2 J 29Si-29Si and 2 J 31P-31P couplings correspond to compounds exhibiting larger Si-O-Si (e.g., 3.6 to 8.0 Hz for angles between 139º and 150º in parawollastonite 98 and 6.3 to 23.5 Hz for angles between 137º 5 and 173º in the zeolite Sigma-2 108 ) and P-O-P (e.g., 9 to 30 Hz for angles between 139º to 157º in (MoO 2 ) 2 P 2 O 7 , see Tables 4 & 5 in Ref. 93 ) bond angles, as compared to the B-O-B bond angles that are in the range 109º to 126º for lithium diborate. The absence of an evident zero crossing due to a J modulation in the experimental spin-echo data for lithium diborate with 25% 11 B abundance in Fig.…”