In the previous article, “Is a Ca/P Ratio Near 1.67 Enough to Confirm HAp?”, we discussed why an average value near the theoretical ratio cannot by itself confirm that a sample contains only HAp.
A value below 1.67 is equally inconclusive on its own. It does not automatically mean that HAp is impure or that calcium-deficient HAp has been confirmed. The value may reflect lattice calcium deficiency, but it may also result from secondary calcium-phosphate phases, amorphous material, ionic substitution, or local sampling.
1.50–1.67 Is a Compositional Range, Not a Naming Shortcut
Ideal stoichiometric HAp has the formula Ca10(PO4)6(OH)2 and a theoretical Ca/P molar ratio of about 1.67. Calcium-deficient hydroxyapatite (CDHA) is commonly represented by the idealized formula:
Ca10−x(HPO4)x(PO4)6−x(OH)2−x, 0 < x ≤ 1
Here, x = 0 represents the stoichiometric HAp end member. Including that boundary, the continuous model spans Ca/P molar ratios from 1.50 to 1.67. In a wet-synthesis study, Raynaud and colleagues produced single-phase apatitic powders within this range. Outside it, a second phase was detected: CaHPO4 below a Ca/P ratio of 1.50 and Ca(OH)2 above 1.667 [1].
The range should not be treated as a new acceptance interval. Ca/P is an elemental-composition result, while CDHA is a conclusion about the material's structural and chemical state.
Calcium-Deficient HAp Is More Than HAp with Less Calcium
A 2026 Acta Biomaterialia literature review treats CDHA as a distinct material class within the Ca–P–O–H system and separates it from ion-substituted apatites and loosely defined “biomimetic apatites” [2].
The review emphasizes a structural and chemical duality comprising a crystalline core and an amorphous hydrated surface layer. That hydrated layer participates in ion exchange and dissolution and affects thermal stability and surface reactivity. Identifying CDHA therefore requires more than measuring how much calcium is missing: the evidence must also show that an apatitic phase remains predominant and clarify how charge balance is maintained.
Four Conditions Can Lower the Ca/P Ratio
- Secondary calcium-phosphate phases: HAp may coexist with phosphate-rich phases or unreacted precursors. The bulk average then falls even if a single apatite lattice is not calcium-deficient.
- Amorphous or poorly ordered components: Amorphous calcium phosphate and hydrated surface material may not produce distinct peaks in routine XRD. Dominant apatite reflections do not show that all calcium and phosphorus occupy one lattice.
- Ionic or carbonate substitution: Cations such as Na, Mg, and Sr, as well as carbonate-related substitution, can alter composition and charge balance. Ion-substituted apatite should not automatically be classified as strictly Ca–P–O–H calcium-deficient HAp.
- Local variability in EDS measurements: Particle size, sampling position, surface condition, and accelerating voltage affect local Ca/P results. EDS does not replace bulk chemical analysis of an entire powder lot [5].
What Evidence Is Needed to Identify CDHA?
| Evidence | What it can answer | What it cannot establish alone |
|---|---|---|
| Bulk calcium and phosphorus analysis | Is the average Ca/P ratio below stoichiometric HAp? | Whether the shift comes from lattice deficiency, secondary phases, or a surface layer |
| SEM-EDS | What is the local elemental composition of a selected particle or region? | The accurate bulk Ca/P ratio and phase identity of the whole powder |
| XRD | Which major crystalline phases and evident secondary phases can be identified? | Low-content, overlapping, or amorphous components |
| FTIR or Raman spectroscopy | Vibrational information associated with HPO42−, PO43−, OH−, and CO32− | The precise amount of each phase |
| Phase and spectroscopic analysis before and after heat treatment | Does heat treatment produce dehydration-related changes, decomposition products, or phase transformation? | The complete surface structure of the original hydrated sample |
| Solid-state NMR | What are the local chemical environments of phosphorus, hydrogen, and other nuclei? | It may not be suitable for routine lot testing |
| XPS | What are the near-surface elemental composition and chemical states? | The bulk composition of the entire powder lot |
A practical sequence begins with bulk chemistry, followed by XRD to identify major crystalline phases and visible secondary phases. FTIR or Raman spectroscopy can then examine groups associated with nonstoichiometric apatite. Samples containing foreign ions, carbonate, or a substantial hydrated layer may require additional compositional and local-structure methods. Drouet's methodological study likewise emphasizes that apatite identity requires complementary evidence [4].
Heat Treatment Changes the Material Being Evaluated
Drying, calcination, and sintering can dehydrate the hydrated layer and alter hydrogen phosphate and hydroxyl states. Under the conditions studied by Raynaud and colleagues, calcium-deficient apatite decomposed at about 700 °C and above to form an HAp/β-TCP biphasic system. The phase proportions depended on the initial Ca/P ratio and the heat-treatment schedule [3]. This temperature was observed for defined powders and heating conditions; it is not a universal transformation point for every CDHA material.
A post-treatment XRD pattern cannot fully reconstruct the surface structure of the original wet precipitate. For HAp materials intended for sintering, spraying, or compounding, a calcium-deficient description at the raw-material stage does not automatically remain valid after processing.
Calcium Deficiency Is Not a Performance Grade
CDHA is studied in bone-repair materials because of its composition, dissolution behavior, and hydrated surface. A lower Ca/P ratio does not automatically make a material more bone-like, and faster dissolution is not inherently better for a given application. The 2026 review summarizes structural, synthetic, in vitro, and in vivo evidence; those trends should not be presented as a clinical-performance claim for a particular raw material [2].
Particle size, morphology, specific surface area, pore structure, crystallinity, and processing still affect material behavior. Calcium deficiency describes a compositional and structural state; it is not, by itself, a quality grade.
How Can a Test Report Describe the Result?
With elemental results only: The measured Ca/P molar ratio is below the theoretical value for stoichiometric HAp. This result alone cannot distinguish lattice calcium deficiency from secondary calcium-phosphate phases, ionic substitution, or sampling variation.
With supporting XRD and spectroscopy: The sample is predominantly apatitic, its bulk Ca/P ratio is below stoichiometric HAp, and compositional features associated with nonstoichiometric apatite are present. The specific defect structure still requires further confirmation.
With a more complete evidence chain: Bulk composition, apatitic structure, control of secondary phases, and local chemical environments support describing the material as calcium-deficient hydroxyapatite under the stated preparation and test conditions.
A Ca/P ratio below 1.67 does not necessarily mean that a material is impure, and it does not automatically prove CDHA. The key question is what phase composition or defect chemistry produced the measured ratio, and whether the data support the term CDHA.
This article discusses material composition and characterization. It does not determine regulatory conformity, clinical use, or the performance of a specific medical product.
References
- Raynaud S, Champion E, Bernache-Assollant D, Thomas P. Calcium phosphate apatites with variable Ca/P atomic ratio I. Synthesis, characterisation and thermal stability of powders. Biomaterials. 2002;23(4):1065–1072. DOI: 10.1016/S0142-9612(01)00218-6.
- Bohner M, Döbelin N, Drouet C, Ginebra MP, Maazouz Y, Marchat D. Calcium-Deficient hydroxyapatite as a bone graft material: From hydrated-layer chemistry to clinical performance. Acta Biomaterialia. 2026;218:1–37. DOI: 10.1016/j.actbio.2026.06.010.
- Raynaud S, Champion E, Bernache-Assollant D. Calcium phosphate apatites with variable Ca/P atomic ratio II. Calcination and sintering. Biomaterials. 2002;23(4):1073–1080. DOI: 10.1016/S0142-9612(01)00219-8.
- Drouet C. Apatite Formation: Why It May Not Work as Planned, and How to Conclusively Identify Apatite Compounds. BioMed Research International. 2013;2013:490946. DOI: 10.1155/2013/490946.
- Miculescu F, Luță C, Constantinescu AE, et al. Considerations and Influencing Parameters in EDS Microanalysis of Biogenic Hydroxyapatite. Journal of Functional Biomaterials. 2020;11(4):82. DOI: 10.3390/jfb11040082.