Absence of charge-charge correlations at intermediate-range distances in disordered network-forming materials

Abstract

Applying a first-principles scheme to liquid GeSe 4 , we show that the charge-charge structure factor does not present any feature at the location of the first sharp diffraction peak ͑FSDP͒, despite a clear FSDP in the concentration-concentration structure factor S CC (k). The origin of this effect is assigned in part to the finite extent of the electron density and in part to electron rearrangements. Our result provides evidence in favor of the postulate that, in binary network-forming systems, charge-charge correlations are absent on intermediaterange length scales. A FSDP in the S CC (k) then signals the occurrence of structural defects.The presence of a first sharp diffraction peak ͑FSDP͒ in the total structure factor of liquid and glasses is a signature of an intermediate-range order ͑IRO͒ established on distances well beyond nearest neighbors. 1-3 The FSDP can be related to specific structural correlations by considering the partial structure factors. 4,5 In particular, a FSDP in the Bhatia-Thornton concentration-concentration partial structure factor S CC (k),is indicative of fluctuations of concentration on distances typical of the IRO. S AA (k), S AX (k), and S XX (k) are the Faber-Ziman partial structure factors for a binary system made of A and X species of concentrations c A and c X , respectively. We note that the presence of a positive ͑negative͒ peak in S CC (k) corresponds to preferred correlations among atoms of the same ͑different͒ kind on length scales associated with the value of k. The absence of a peak reflects insensitivity to the chemical nature of the neighbors. On the basis of classical molecular-dynamics simulations, it was postulated that the absence of a FSDP in the structure factor S zz (k) is a general feature of any AX 2 disordered network, independently of the adopted theoretical scheme. 6,7 This statement appeared undermined by neutron-diffraction measurements which showed a FSDP in the structure factor S CC (k) of liquid GeSe 2 (l-GeSe 2 ). 8,9 In fact, under the assumption of pointlike ionic charges ͑as is the case in classical molecular dynamics͒, the structure factor S CC (k) becomes trivially proportional to the charge-charge structure factor S zz (k). However, for real systems the pointlike ionic model is only an approximation, and S CC (k) and S zz (k) are no longer necessarily equivalent. Hence, the postulation that charge-charge fluctuations are absent on intermediate-range distances might still have a profound meaning. The situation became even more intriguing when first-principles molecular-dynamics simulations of l-GeSe 2 , which properly account for charge transfer, also did not show any FSDP in the structure factor S CC (k), yet exhibiting excellent agreement for the total neutron structure factor over the entire k range. 10 In this work, we investigate the relation between chargecharge correlations and concentration-concentration correlations for a realistic system. We selected liquid GeSe 4 because previous simulations have shown that l-GeSe 4 presents a FSDP in t...

Charge fluctuations and concentration fluctuations at intermediate-range distances in the disordered network-forming materialsSiO2,SiMassobrio1, Celino2, Pasquarello3 2004Phys. Rev. BSelf Cite

“…These systems are representative of the three classes (l-SiO 2 : class I, a-SiSe 2 : class II, l-GeSe 2 : class III) which were introduced to relate the appearance of a FSDP in the S CC ͑k͒ to the different degrees of departure from chemical order. 23 Our calculations reveal that no FSDP appears in any of the calculated charge-charge structure factors S zz ͑k͒. These results provide evidence in support of the postulate advanced in Ref.…”

“…25 This class is consistent with very moderate departures from chemical order, as confirmed by the structure of l-GeSe 4 and l-SiSe 2 obtained by simulation. 23,26,27 A network structure of class III has so far only been encountered in first-principles molecular dynamics simulations of l-GeSe 2 . 18 Like for class I, no feature appears at the FSDP location in the S CC ͑k͒.…”

“…We were able to rationalize the occurrence of a FSDP in the S CC ͑k͒ as a signal for the departure from chemical order, and to identify systems of three different classes. 23 Class I encompasses networks showing perfect chemical order and the absence of any FSDP in the S CC ͑k͒. This class includes systems like SiO 2 and GeO 2 , which have been characterized either experimentally or by simulation.…”

“…28 In a recent paper, we carried out first-principles molecular dynamics simulations on l-GeSe 4 . 23 Our purpose was twofold. First, we wanted to investigate the origin of the FSDP which appeared in the S CC ͑k͒ of this system.…”

Intermediate-range order in molten network-forming systems

“…The conclusion was that charge correlations in the region of IRO are absent and coordination defects were responsible. 62 Replication of the FSDP in S CC (k) has proved problematical in computational methods. Chapter1.…”