The high temperature structures of quartz and cristobalite have a structure in which the oxygen atoms are statistically disordered with 1/6 occupation of O sites in what is probably a micro-twinned structure which preserves the observed high-symmetry crystal structure whilst enabling a lower-symmetry local structure consistent with accepted bonding relations. This has been demonstrated in high quartz (Wright & Lehmann 1981) and in the high cristobalite phases of Silica and AlPO4 (Wright & Leadbetter 1975, Leadbetter, Smith & Wright 1973). Although this work is not recent, (mostly from my theses work) it still attracts interest and citations today. There is still work to be done on quartz at the low-high transition and on the high quartz phase of AlPO4 and analogues. I'm open to any proposals, esp. from crystallographers.
Best regards,
Albert Wright
Hi, Albert!
What exactly do you need help with? I could offer to do a full structure database search for starters. :D
regards,
Daniel
Hi Daniel,
Thank you for your offer. That would be a good start. For silica quartz when we (wright & lehmann) were making the high quartz study, we were following peaks of low quartz just below the transition when we suddenly lost all signals, only to find them again a few degrees higher in the high quartz phase. We later found the explanation in Dolino's work relating to an incommensurate phase existing briefly at the transition. Might also be worth revisiting with diffraction if not already done. Since I'm retired I don't have priority access to lab facilities or online publications anymore, but still have ideas.
Best regards
Albert Wright
In case of an incommensurate structure...do you really loose the diffraction signal? I mean, this is the consequence of a loss of first order bondings (no coherence anymore)...whereas incommensurate structures have only translation symmetry which is described in higher dimensions than 2 or three, i.e. they form theoretically sharp interferences... Or am I wrong?
Thanks for your question Gert.. when we lost the diffraction signal, I don't mean there was none, just that we didn't know where to find the reflections due to the limitations of the instrument in those days (1984) We were working on a high resolution neutron 4-circle with a single detector (no 2-D detection) and no time to do a survey. When we found the beta phase we carried on as scheduled.
In the meantime Gerard Dolino sent me a message which is relevant so I copy it below. (in French)
Cher Albert, bonjour,
Avec Pierre Bastie nous exploitons des résultats obtenus sur le quartz à l'ESRF de 2004 à 2008. Un article sur la coexistence des phases alpha et incomensurable devrait être soumis dans quelques mois.
Nous avons fait quelques mesures sur AlPO4 (1 seule publication sur la phase inc, Bachheimer et al, Solid State Com; 51, 55-58, 1984). La mauvaise qualité des cristaux de l'époque nous a fait abandonné ce cristal. Je pense qu'on a renoncé à utiliser ce cristal pour remplacer le quartz? As-tu des informations à ce sujet?
Bien cordialement
Gérard Dolino.
In reply to Gerard's question. Many years ago I was able to grow very beautiful AlPO4 quartz crystals syntheticaly using a hydrothermal bomb. As I recall it was not difficult and took just a few weeks. of course I don't have them any more.
Hi again, Albert!
Before I delve any deeper into the crystal data, I have summed the main families up in some sort of table: quartz (SiO2, alpha and beta) and berlinite (AlPO4), see pdf attachment. Am I on the right track? If so, what kind of data summary would you like? Diffraction patterns? Bond distances? Venn diagrams? ; )
regards,
Daniel
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