How do I know crystal structures from formula?












1















Give an explanation why $ce{MgF2}$ and $ce{CaF2}$ adopt different structure types.




For example, the structure of calcium fluoride is 'fluorite' and the structure of magnesium fluoride is 'rutile'. I am struggling, just from looking at the formulae, on how I determine this?






inorganic-chemistry crystal-structure solid-state-chemistry







share|improve this question




















  • 3




    You don't. That being said, a post factum explanation is somewhat possible.
    – Ivan Neretin
    4 hours ago
















1















Give an explanation why $ce{MgF2}$ and $ce{CaF2}$ adopt different structure types.




For example, the structure of calcium fluoride is 'fluorite' and the structure of magnesium fluoride is 'rutile'. I am struggling, just from looking at the formulae, on how I determine this?






inorganic-chemistry crystal-structure solid-state-chemistry







share|improve this question




















  • 3




    You don't. That being said, a post factum explanation is somewhat possible.
    – Ivan Neretin
    4 hours ago














1












1








1








Give an explanation why $ce{MgF2}$ and $ce{CaF2}$ adopt different structure types.




For example, the structure of calcium fluoride is 'fluorite' and the structure of magnesium fluoride is 'rutile'. I am struggling, just from looking at the formulae, on how I determine this?






inorganic-chemistry crystal-structure solid-state-chemistry







share|improve this question
















Give an explanation why $ce{MgF2}$ and $ce{CaF2}$ adopt different structure types.




For example, the structure of calcium fluoride is 'fluorite' and the structure of magnesium fluoride is 'rutile'. I am struggling, just from looking at the formulae, on how I determine this?






inorganic-chemistry crystal-structure solid-state-chemistry




inorganic-chemistry crystal-structure solid-state-chemistry






share|improve this question















share|improve this question













share|improve this question




share|improve this question








edited 1 hour ago









andselisk

13.9k648103




13.9k648103










asked 5 hours ago









KnightKnight

161




161








  • 3




    You don't. That being said, a post factum explanation is somewhat possible.
    – Ivan Neretin
    4 hours ago














  • 3




    You don't. That being said, a post factum explanation is somewhat possible.
    – Ivan Neretin
    4 hours ago








3




3




You don't. That being said, a post factum explanation is somewhat possible.
– Ivan Neretin
4 hours ago




You don't. That being said, a post factum explanation is somewhat possible.
– Ivan Neretin
4 hours ago










2 Answers
2






active

oldest

votes


















3














I'll just point out the direction in which you could look for an answer.



In general case, it should be almost impossible to determine the crystal structure by looking at the chemical composition of a substance.



You are, however, asked to explain why different fluorides adopt different structure types, which is a different task. Perhaps, you should take into account the difference (sorry) in the ionic radii of Mg and Ca. Some structures prefer "fat" cations, some - "lean" ones. Most of the time, it's just plain geometry combined with experimental observations. I know the explanation is primitive, and ions are not just some rigid spheres, but the "ionic radii" approach works in many cases.



For many crystal structures there are ranges of ionic radii ratios in which the particular structure is more or less stable. Some of these ratios are so important that they have proper names, such as Goldschmidt tolerance factor for perovskites. Returning to your example, this page lists the cation/anion radii ratios for fluorite (0.73 or above) and rutile (between 0.41 and 0.73). You can check these ratios against the dimensions of ions in magnesium and calcium fluorites.



For the reference - this Shannon table hosted by Imperial College London is an excellent source of the values of ionic radii in oxides and halides. Just one more thing: the reference ratios are likely to be designed to work with so-called Shannon Ionic Radii, not the less-common Shannon Crystal Radii.






share|improve this answer





























    1














    Not really a complete answer, rather an addition to the given answer (which I think you should accept). As of now, it's possible to predict existence of certain compounds, mostly binary and ternary inorganic ionic solids, at the given temperature and pressure, as well as their crystal structure.



    There are many computer programs, among which USPEX is arguably the most feature-rich and promoted. It uses Oganov-Glass evolutionary algorithm and works best for network solids (including particles and clusters). Fairly recently there also were some advancements for predicting crystal structures of small organic molecules.






    share|improve this answer



















    • 1




      Suggestion: The blind tests moderated by the CCDC you mention advanced further than to the fourth round your link points to (cf. ccdc.cam.ac.uk/Community/initiatives/cspblindtests). Which is why I'm going to replace the link set by you by the one pointing to the sixth one -- equally published as open access article on Acta Cryst B.
      – Buttonwood
      42 mins ago










    • @Buttonwood Good catch, thank you for the edit!
      – andselisk
      39 mins ago






    • 1




      You knew better about USPEX, and me a bit about the contests. It's one of the enjoyable ideas of SE, bringing the knowledge of multiple people into one forum, an exchange.
      – Buttonwood
      24 mins ago













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    2 Answers
    2






    active

    oldest

    votes








    2 Answers
    2






    active

    oldest

    votes









    active

    oldest

    votes






    active

    oldest

    votes









    3














    I'll just point out the direction in which you could look for an answer.



    In general case, it should be almost impossible to determine the crystal structure by looking at the chemical composition of a substance.



    You are, however, asked to explain why different fluorides adopt different structure types, which is a different task. Perhaps, you should take into account the difference (sorry) in the ionic radii of Mg and Ca. Some structures prefer "fat" cations, some - "lean" ones. Most of the time, it's just plain geometry combined with experimental observations. I know the explanation is primitive, and ions are not just some rigid spheres, but the "ionic radii" approach works in many cases.



    For many crystal structures there are ranges of ionic radii ratios in which the particular structure is more or less stable. Some of these ratios are so important that they have proper names, such as Goldschmidt tolerance factor for perovskites. Returning to your example, this page lists the cation/anion radii ratios for fluorite (0.73 or above) and rutile (between 0.41 and 0.73). You can check these ratios against the dimensions of ions in magnesium and calcium fluorites.



    For the reference - this Shannon table hosted by Imperial College London is an excellent source of the values of ionic radii in oxides and halides. Just one more thing: the reference ratios are likely to be designed to work with so-called Shannon Ionic Radii, not the less-common Shannon Crystal Radii.






    share|improve this answer


























      3














      I'll just point out the direction in which you could look for an answer.



      In general case, it should be almost impossible to determine the crystal structure by looking at the chemical composition of a substance.



      You are, however, asked to explain why different fluorides adopt different structure types, which is a different task. Perhaps, you should take into account the difference (sorry) in the ionic radii of Mg and Ca. Some structures prefer "fat" cations, some - "lean" ones. Most of the time, it's just plain geometry combined with experimental observations. I know the explanation is primitive, and ions are not just some rigid spheres, but the "ionic radii" approach works in many cases.



      For many crystal structures there are ranges of ionic radii ratios in which the particular structure is more or less stable. Some of these ratios are so important that they have proper names, such as Goldschmidt tolerance factor for perovskites. Returning to your example, this page lists the cation/anion radii ratios for fluorite (0.73 or above) and rutile (between 0.41 and 0.73). You can check these ratios against the dimensions of ions in magnesium and calcium fluorites.



      For the reference - this Shannon table hosted by Imperial College London is an excellent source of the values of ionic radii in oxides and halides. Just one more thing: the reference ratios are likely to be designed to work with so-called Shannon Ionic Radii, not the less-common Shannon Crystal Radii.






      share|improve this answer
























        3












        3








        3






        I'll just point out the direction in which you could look for an answer.



        In general case, it should be almost impossible to determine the crystal structure by looking at the chemical composition of a substance.



        You are, however, asked to explain why different fluorides adopt different structure types, which is a different task. Perhaps, you should take into account the difference (sorry) in the ionic radii of Mg and Ca. Some structures prefer "fat" cations, some - "lean" ones. Most of the time, it's just plain geometry combined with experimental observations. I know the explanation is primitive, and ions are not just some rigid spheres, but the "ionic radii" approach works in many cases.



        For many crystal structures there are ranges of ionic radii ratios in which the particular structure is more or less stable. Some of these ratios are so important that they have proper names, such as Goldschmidt tolerance factor for perovskites. Returning to your example, this page lists the cation/anion radii ratios for fluorite (0.73 or above) and rutile (between 0.41 and 0.73). You can check these ratios against the dimensions of ions in magnesium and calcium fluorites.



        For the reference - this Shannon table hosted by Imperial College London is an excellent source of the values of ionic radii in oxides and halides. Just one more thing: the reference ratios are likely to be designed to work with so-called Shannon Ionic Radii, not the less-common Shannon Crystal Radii.






        share|improve this answer












        I'll just point out the direction in which you could look for an answer.



        In general case, it should be almost impossible to determine the crystal structure by looking at the chemical composition of a substance.



        You are, however, asked to explain why different fluorides adopt different structure types, which is a different task. Perhaps, you should take into account the difference (sorry) in the ionic radii of Mg and Ca. Some structures prefer "fat" cations, some - "lean" ones. Most of the time, it's just plain geometry combined with experimental observations. I know the explanation is primitive, and ions are not just some rigid spheres, but the "ionic radii" approach works in many cases.



        For many crystal structures there are ranges of ionic radii ratios in which the particular structure is more or less stable. Some of these ratios are so important that they have proper names, such as Goldschmidt tolerance factor for perovskites. Returning to your example, this page lists the cation/anion radii ratios for fluorite (0.73 or above) and rutile (between 0.41 and 0.73). You can check these ratios against the dimensions of ions in magnesium and calcium fluorites.



        For the reference - this Shannon table hosted by Imperial College London is an excellent source of the values of ionic radii in oxides and halides. Just one more thing: the reference ratios are likely to be designed to work with so-called Shannon Ionic Radii, not the less-common Shannon Crystal Radii.







        share|improve this answer












        share|improve this answer



        share|improve this answer










        answered 3 hours ago









        voffchvoffch

        3455




        3455























            1














            Not really a complete answer, rather an addition to the given answer (which I think you should accept). As of now, it's possible to predict existence of certain compounds, mostly binary and ternary inorganic ionic solids, at the given temperature and pressure, as well as their crystal structure.



            There are many computer programs, among which USPEX is arguably the most feature-rich and promoted. It uses Oganov-Glass evolutionary algorithm and works best for network solids (including particles and clusters). Fairly recently there also were some advancements for predicting crystal structures of small organic molecules.






            share|improve this answer



















            • 1




              Suggestion: The blind tests moderated by the CCDC you mention advanced further than to the fourth round your link points to (cf. ccdc.cam.ac.uk/Community/initiatives/cspblindtests). Which is why I'm going to replace the link set by you by the one pointing to the sixth one -- equally published as open access article on Acta Cryst B.
              – Buttonwood
              42 mins ago










            • @Buttonwood Good catch, thank you for the edit!
              – andselisk
              39 mins ago






            • 1




              You knew better about USPEX, and me a bit about the contests. It's one of the enjoyable ideas of SE, bringing the knowledge of multiple people into one forum, an exchange.
              – Buttonwood
              24 mins ago


















            1














            Not really a complete answer, rather an addition to the given answer (which I think you should accept). As of now, it's possible to predict existence of certain compounds, mostly binary and ternary inorganic ionic solids, at the given temperature and pressure, as well as their crystal structure.



            There are many computer programs, among which USPEX is arguably the most feature-rich and promoted. It uses Oganov-Glass evolutionary algorithm and works best for network solids (including particles and clusters). Fairly recently there also were some advancements for predicting crystal structures of small organic molecules.






            share|improve this answer



















            • 1




              Suggestion: The blind tests moderated by the CCDC you mention advanced further than to the fourth round your link points to (cf. ccdc.cam.ac.uk/Community/initiatives/cspblindtests). Which is why I'm going to replace the link set by you by the one pointing to the sixth one -- equally published as open access article on Acta Cryst B.
              – Buttonwood
              42 mins ago










            • @Buttonwood Good catch, thank you for the edit!
              – andselisk
              39 mins ago






            • 1




              You knew better about USPEX, and me a bit about the contests. It's one of the enjoyable ideas of SE, bringing the knowledge of multiple people into one forum, an exchange.
              – Buttonwood
              24 mins ago
















            1












            1








            1






            Not really a complete answer, rather an addition to the given answer (which I think you should accept). As of now, it's possible to predict existence of certain compounds, mostly binary and ternary inorganic ionic solids, at the given temperature and pressure, as well as their crystal structure.



            There are many computer programs, among which USPEX is arguably the most feature-rich and promoted. It uses Oganov-Glass evolutionary algorithm and works best for network solids (including particles and clusters). Fairly recently there also were some advancements for predicting crystal structures of small organic molecules.






            share|improve this answer














            Not really a complete answer, rather an addition to the given answer (which I think you should accept). As of now, it's possible to predict existence of certain compounds, mostly binary and ternary inorganic ionic solids, at the given temperature and pressure, as well as their crystal structure.



            There are many computer programs, among which USPEX is arguably the most feature-rich and promoted. It uses Oganov-Glass evolutionary algorithm and works best for network solids (including particles and clusters). Fairly recently there also were some advancements for predicting crystal structures of small organic molecules.







            share|improve this answer














            share|improve this answer



            share|improve this answer








            edited 35 mins ago

























            answered 1 hour ago









            andseliskandselisk

            13.9k648103




            13.9k648103








            • 1




              Suggestion: The blind tests moderated by the CCDC you mention advanced further than to the fourth round your link points to (cf. ccdc.cam.ac.uk/Community/initiatives/cspblindtests). Which is why I'm going to replace the link set by you by the one pointing to the sixth one -- equally published as open access article on Acta Cryst B.
              – Buttonwood
              42 mins ago










            • @Buttonwood Good catch, thank you for the edit!
              – andselisk
              39 mins ago






            • 1




              You knew better about USPEX, and me a bit about the contests. It's one of the enjoyable ideas of SE, bringing the knowledge of multiple people into one forum, an exchange.
              – Buttonwood
              24 mins ago
















            • 1




              Suggestion: The blind tests moderated by the CCDC you mention advanced further than to the fourth round your link points to (cf. ccdc.cam.ac.uk/Community/initiatives/cspblindtests). Which is why I'm going to replace the link set by you by the one pointing to the sixth one -- equally published as open access article on Acta Cryst B.
              – Buttonwood
              42 mins ago










            • @Buttonwood Good catch, thank you for the edit!
              – andselisk
              39 mins ago






            • 1




              You knew better about USPEX, and me a bit about the contests. It's one of the enjoyable ideas of SE, bringing the knowledge of multiple people into one forum, an exchange.
              – Buttonwood
              24 mins ago










            1




            1




            Suggestion: The blind tests moderated by the CCDC you mention advanced further than to the fourth round your link points to (cf. ccdc.cam.ac.uk/Community/initiatives/cspblindtests). Which is why I'm going to replace the link set by you by the one pointing to the sixth one -- equally published as open access article on Acta Cryst B.
            – Buttonwood
            42 mins ago




            Suggestion: The blind tests moderated by the CCDC you mention advanced further than to the fourth round your link points to (cf. ccdc.cam.ac.uk/Community/initiatives/cspblindtests). Which is why I'm going to replace the link set by you by the one pointing to the sixth one -- equally published as open access article on Acta Cryst B.
            – Buttonwood
            42 mins ago












            @Buttonwood Good catch, thank you for the edit!
            – andselisk
            39 mins ago




            @Buttonwood Good catch, thank you for the edit!
            – andselisk
            39 mins ago




            1




            1




            You knew better about USPEX, and me a bit about the contests. It's one of the enjoyable ideas of SE, bringing the knowledge of multiple people into one forum, an exchange.
            – Buttonwood
            24 mins ago






            You knew better about USPEX, and me a bit about the contests. It's one of the enjoyable ideas of SE, bringing the knowledge of multiple people into one forum, an exchange.
            – Buttonwood
            24 mins ago




















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