Solid Solution and Crystal Engineering

Deraz NM

Published on: 2019-09-18


The full success of crystal engineering will have a positive impact in our technological development. Solid solutions are one of the methods of crystal engineering. These solutions were divided into the substitutional and interstitial solid solutions. However, these solutions have the potential to transform the field of crystal engineering to applicable fields. In addition, the study of such solutions answers an aesthetic demand that is implicit in each scientific activity. Besides the traditional work on metals (alloys) and inorganic salts, many examples of solid solutions include pharmaceuticals, functional materials, inclusion compounds, and porous solids. Solid solutions can be exploited to enforce a desired molecular connectivity and crystal symmetry and to fine-tune unit cell dimension as well as many other physicochemical properties. In some cases, solid solutions of different materials can produce exotic characteristics that are not observed in the parent solids. In other cases, solid solutions are reported as an inconvenient phenomenon that needs to be avoided. Examples of the latter include purification, ex-solution or resolution, and molecular separation processes. Solid solutions can be characterized by various techniques such as XRD, XRF, SEM and TEM. There are different preparation methods for such solutions. Mechanisms of solid solutions were determined.


Substitutional and interstitial solid solutions; XRD; Preparation; Mechamism and applications


The Solutions play an important and effective role in our daily lives. The tea, coffee, soft drinks and perfumes that you enjoy daily are solutions. A solution is a special type of mixture composed of two or more substances. In such a mixture, a solute (A) is a substance dissolved in another substance, known as a solvent (B). A is present in a smaller quantity. B is present in a comparatively larger quantity. There are various types of solutions with subsequent different characteristics. Solutions are of different types, based on a number of criteria like the difference in the type and nature of solute or the solvent. Based on the whether the solvent is water or not, solutions are aqueous solutions and non-Aqueous solutions. They are unsaturated solutions, saturated solutions and supersaturated solutions Based on the amount of solute present in the solution. Amount of the solvent added brought about concentrated Solutions and dilute solutions. On the other hand, Concentration of Solute in Two Solutions resulted in isotonic Solution, Hypertonic solution and hypotonic solution. In addition, dissolution of gases, liquids and solids resulted in gaseous, liquid and solid solutions, respectively. Hydrogen, mercury and carbon can be dissolved in palladium, gold and a crystalline matrix of iron atoms forming hydrogen storage solid, an amalgam and steel, respectively. This report aims to talk about solid solutions and their impact in the industrial and environmental fields.

Solid solutions and their characteristics

Solid solution is a solid state solution of one or more solutes solid in a solvent solid. In other words, it is composed of only one phase when the solute and solvent are close together on the periodic table [1-2]. It is characterized by the fact that the crystal structure of the solvent remains unchanged and the chemical constituents remain in a single homogeneous phase. However, the solution has the same physical state of the solvent.

Preparation Methods of Solid Solutions

Preparation methods with all their variables are considered one of the most important factors affecting the solid solution formation. Solid state synthesis is the classical method of preparation of solid solutions [3]. Significant disadvantages of this method are the necessity of using high temperatures for long time with subsequent increase in the sinterability and heterogeneity of the as prepared solids. Thus, an important task is to find alternative methods of "soft” chemistry. The most common methods of the synthesis of solid solutions are co-precipitation, micro emulsion, hydro- and solvothermal routes….etc [4-5]. Another interesting method of synthesis is auto combustion of the precursors yielding the desired solid solutions. Previously, we have shown the effectiveness of this approach for production of ferrite based nano materials with unique properties such as structural, morphemically, surface and catalytic characteristics [6-7].

Characterization techniques of solid solutions

Different techniques can be used for characterization of solid solutions such as Thermogravimetry/Differential Thermal Analysis (TG/DTA), X-ray diffraction (XRD), X-ray fluorescence (XRF), transmission electron microscopy (TEM) and scanning Electron microscopy (SEM)…..etc. In fact, XRD is one of the most important techniques for identifying the formation of a solid solution as well as all the important data which gives about the structural properties of the as synthesized solid solution.

Mechanisms Of Solid Solution Fabrication

There are two mechanisms to insert the solute into the solvent crystal according to Hume- Rothery rules as follows:

Substitution mechanism

It can be achieved by replacing some solvent particles by solute particles. This mechanism resulted in substitutional crystals and partially occupied host/guest systems [8]. For substitutional solid solutions, the Hume-Rothery rules are:

  1. The atomic radii of the solute and solvent atoms must differ by no more than 15%:
  2. The crystal structures of solute and solvent must match.
  3. Maximum solubility occurs when the solvent and solute have the same valence. On the other hand, the metals with lower valence will tend to dissolve metals with higher valence because the last metals have low ionic radii.
  4. The solute and solvent should have similar electronegativity. On the other hand, if the electronegativity difference is too great, the metals will tend to form intermetallic compounds instead of solid solutions. This confirms the transformational phenomena yielding intermetallic compounds with subsequent completely new lattice. This indicates that some solute particles substitute some solvent particles in the crystal lattice without structural changes. Substitutional solid solutions can be random (Cu-Ni) or ordered (Cu-Au).

Interstitial mechanism

 It can be occurred by incorporating of some solute particles into the space between solvent particles. This mechanism brought about interstitial and intercalated solids such as Steel – C solute atoms in Fe [9]. For interstitial solid solutions, the Hume-Rothery rules are:

  1. Solute atoms must be smaller than the pores in the solvent lattice.
  2. The solute and solvent should have similar electronegativity.

This indicates the solute does not occupy the sites in the lattice of the solvent but resides in the crystallographic pores.

Technologically Relevant Properties

Solid solutions have been studied for almost a century to investigate the effect of chemical composition on optical, electrical, and photochemical properties [10]. Polycrystalline of Zn1-xMgxO (0≤x≤0.36) have been prepared by a sol-gel method. The authors investigate the effects of the Mg addition on crystallization, microstructure and optical properties for ZnO preheated at 300oC for 10 min and then annealed at 500oC for 1 h. The results showed Mg was incorporated into ZnO with subsequent decease in the surface roughness. This treatment resulted in an improved transparency in the visible range and an increase in the resistivity. Among the Zn1-xMgxO studied, the Zn0.8Mg0.2O exhibited the best properties, namely single wurzite phase, an optical transmittance of 94.7%, root mean square deviation (RMS roughness) of 1.63 nm and a resistivity of 8.3 x 105 ?-cm. Deraz ' scientific group reported to incorporation of lithium in the crystal lattices of CuO/CeO2 catalysts and nanomagnetic zinc ferrite synthesized by impregnation and combustion methods, respectively. This group confirms that this incorporation led to significant modifications in the structural, morphological, surface, magnetic and catalytic properties [6-7]. On the other hand, Deraz group was investigated the magnesium ferrite spinel solid solution with iron -rich and magnesium rich compositions prepared by combustion route [11-12].

Applications Of Solid Solutions

Solid solutions have important commercial and industrial applications; as such mixtures often have superior properties to pure materials. Many metal alloys are solid solutions. Even small amounts of solute can affect the electrical, thermal, mechanical and physical properties of the solvent. The following table contains some examples as solid solutions of some metals and their various applications [13].

Alloys or solid solutions




Hg with other metals

Mercury is used for extraction of metals like silver and gold from their ores. These metals are further recovered by distillation of mercury.


84 % Cu, 12 % Mn and 4% Ni

Electrical measurement instrument due to zero temperature coefficient of resistance.


Aluminum, copper, manganese and magnesium

Aircraft, boats, railroad cars, and machinery because of its high strength and resistance to corrosion

Aluminum bronze

Aluminum, copper and




Heavy duty sleeve bearings, and machine tool ways. Aluminum bronze castings have exceptional corrosion resistance, high strength, toughness and wear resistance and good casting and welding characteristics

Stainless Steel

Iron, chromium and nickel

Cutlery, surgical instruments, utensils


Iron and 5 to 20 % manganese

Rails, safes and heavy machinery


Iron and 70 to 80 % manganese

Rails, safes and heavy machinery


The control of structures and properties in crystalline materials has many returns that justify the increasing efforts in this direction via different preparation methods. Recently, reports on crystalline solid solutions have become common in crystal engineering research. Crystalline solid solutions are characterized by a structural disorder that enables the variation of stoichiometry in continuum. Often such variation corresponds to a variation of structural and physicochemical properties, and offers an opportunity for the materials’ fine-tuning. These findings resulted in new and unexpected properties of the substitutional and interstitial solid solutions as prepared materials. Finally, the author believes that solid solutions are big treasure required further studies. The author also believes that solid solutions can be considered as one of the pillars of crystal engineering.