2.2 Loading Methods
2.2.1 Low-Melting Compounds
Some of the early work in single-crystal high-pressure crystallography investigated
the difference in the crystallisation behaviour of liquids under cooling or through
compression. Low-melting compounds are still being investigated as they simplify
the nature of interactions between molecules so that, for example, the effect of
halogen bonding may be investigated more specifically [12, 13]. The preparation
for the loading of low-melting compounds is no different to any other high-pressure
loading and, in some ways, a little easier as there are fewer items requiring loading,
i.e. ruby and liquid. There are a few considerations that need to be taken into account
for a successful experiment such as the maximum pressure of the experiment to be
conducted and the volatility of the liquid in question. For most one-off crystallisation
experiments at lower pressures, gasket holes with a large diameter (250–300 μm)
and greater depth (100–120 μm) permit a larger crystal to be grown. This is
advantageous as it provides potentially stronger diffraction from the crystal.
However, it does limit the maximum pressure at which the sample can be analysed,
but, in most cases in the literature, a maximum of 1.5 GPa would be the norm as the
melting curve generally increases with pressure thereby limiting the maximum
pressure at which the liquid can be heat annealed. To load the cell, a ruby chip is
placed in the sample chamber (or in the opposing diamond) and a drop of the liquid
under study is placed over the hole ensuring that there are no significant bubbles
present in the chamber; the ease of this process will depend on the volatility of the
liquid. Through trial and error, for the best success and to allow for some evaporation, use the surface tension of the liquid to maximise the liquid on the gasket surface
which also provides extra time to deal with any bubbles that may be present in the
hole. Once the cell is sealed, there may be a small bubble present, but usually these
are solubilised on compression.
Nucleation of products at pressure can be troublesome as liquids can form glasses
on compression and the pressure increases that are used to initiate the crystallisation
can be rapid; hence, kinetics can play a significant role. Nevertheless nucleation can
occur through a number of different methods from compression of the liquid alone
[14], pressure cycling (varying pressure up and down) [15], through use of liquid
nitrogen to freeze the sample [16] or applying heat [17]. Each of these methods has
successfully been used to nucleate samples, and one may need to explore all of these
methods for a successful outcome. Further recent methods will be explored when we
discuss quenching and recovery of samples (Sect. 2.3.2.1).
In general, the nucleation product is polycrystalline which can be analysed using
X-ray powder diffraction or spectroscopic methods, but for those who wish to
perform single-crystal diffraction, there is a requirement to anneal the polycrystalline
material into one crystal or a reduced number of crystallites for analysis. Two main
methods used are heat annealing and pressure annealing. Both methods require
that the sample is close to the melting line of the compound which may not
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S. A. Moggach and I. D. H. Oswald
2.2.1 Low-Melting Compounds
Some of the early work in single-crystal high-pressure crystallography investigated
the difference in the crystallisation behaviour of liquids under cooling or through
compression. Low-melting compounds are still being investigated as they simplify
the nature of interactions between molecules so that, for example, the effect of
halogen bonding may be investigated more specifically [12, 13]. The preparation
for the loading of low-melting compounds is no different to any other high-pressure
loading and, in some ways, a little easier as there are fewer items requiring loading,
i.e. ruby and liquid. There are a few considerations that need to be taken into account
for a successful experiment such as the maximum pressure of the experiment to be
conducted and the volatility of the liquid in question. For most one-off crystallisation
experiments at lower pressures, gasket holes with a large diameter (250–300 μm)
and greater depth (100–120 μm) permit a larger crystal to be grown. This is
advantageous as it provides potentially stronger diffraction from the crystal.
However, it does limit the maximum pressure at which the sample can be analysed,
but, in most cases in the literature, a maximum of 1.5 GPa would be the norm as the
melting curve generally increases with pressure thereby limiting the maximum
pressure at which the liquid can be heat annealed. To load the cell, a ruby chip is
placed in the sample chamber (or in the opposing diamond) and a drop of the liquid
under study is placed over the hole ensuring that there are no significant bubbles
present in the chamber; the ease of this process will depend on the volatility of the
liquid. Through trial and error, for the best success and to allow for some evaporation, use the surface tension of the liquid to maximise the liquid on the gasket surface
which also provides extra time to deal with any bubbles that may be present in the
hole. Once the cell is sealed, there may be a small bubble present, but usually these
are solubilised on compression.
Nucleation of products at pressure can be troublesome as liquids can form glasses
on compression and the pressure increases that are used to initiate the crystallisation
can be rapid; hence, kinetics can play a significant role. Nevertheless nucleation can
occur through a number of different methods from compression of the liquid alone
[14], pressure cycling (varying pressure up and down) [15], through use of liquid
nitrogen to freeze the sample [16] or applying heat [17]. Each of these methods has
successfully been used to nucleate samples, and one may need to explore all of these
methods for a successful outcome. Further recent methods will be explored when we
discuss quenching and recovery of samples (Sect. 2.3.2.1).
In general, the nucleation product is polycrystalline which can be analysed using
X-ray powder diffraction or spectroscopic methods, but for those who wish to
perform single-crystal diffraction, there is a requirement to anneal the polycrystalline
material into one crystal or a reduced number of crystallites for analysis. Two main
methods used are heat annealing and pressure annealing. Both methods require
that the sample is close to the melting line of the compound which may not
144
S. A. Moggach and I. D. H. Oswald